Lord Viṣṇu's impersonal aspect is known as Brahman. So when the jīva soul, a product of Lord Viṣṇu's superior, spiritual energy, attains sāyujya-mukti, or liberation by merging with Brahman, it is not at all surprising. The energetic principle always enjoys the prerogative of enfolding within itself His own energy, but that does not destroy the energy's eternal individuality. The impersonalists, desiring to merge with Brahman and knowing that it is feasible, still experience intense suffering in their effort to reach brahmānanda, "the bliss of Brahman." The Lord's devotees consider the pleasures of such liberation worse than hell. The impersonalists, in trying to destroy the illusion inherent in material forms, do away with even the eternal spiritual forms. That is indeed very foolish. Treating a patient to cure his disease is one thing, but ending the patient along with the disease is the work of an idiot. Thus we have this instruction from the great authority Brahmā in the Śrīmad-Bhāgavatam (10.14.4):
śreya-sṛtiṁ bhaktim udasya te vibho
kliśyanti ye kevala-bodha-labdhaye
teṣām asau kleśala eva śiṣyate
nānyad yathā sthūla-tuṣāvaghātinām
My dear Lord, devotional service unto You is the best path for self-realization. If one gives up that path and engages in the cultivation of speculative knowledge, he will simply undergo a troublesome process and will not achieve his desired result. As a person who beats an empty husk of wheat cannot get grain, one who simply speculates cannot achieve self-realization. His only gain is trouble.
(Renunciation through Wisdom).

When Lord Kṛṣṇa, along with His elder brother Balarāma and sister Subhadrā, came to Kurukṣetra in a chariot on the occasion of a solar eclipse, many mystic yogīs also came. When these mystic yogīs saw Lord Kṛṣṇa and Balarāma, they exclaimed that now that they had seen the excellent bodily effulgence of the Lord, they had almost forgotten the pleasure derived from impersonal Brahman realization. In this connection one of the mystics approached Kṛṣṇa and said, "My dear Lord, You are always full with transcendental bliss, excelling all other spiritual positions. And so, simply by seeing You from a distant place, I have come to the conclusion that there is no need of my being situated in the transcendental bliss of impersonal Brahman."
When a great mystic was once awakened from his meditative trance by hearing the vibration of Kṛṣṇa's Pāñcajanya conchshell, the mystic became overpowered—so much so, in fact, that he began to bash his head on the ground, and with eyes full of tears of ecstatic love, he violated all the rules and regulations of his yoga performances. Thus he at once neglected the process of Brahman realization.
(Nectar of Devotion).

"As long as there is not the slightest fragrance of pure love of Kṛṣṇa, which is the perfect medicinal herb for controlling Lord Kṛṣṇa within the heart, the opulences of the material perfections known as the siddhis, the brahminical perfections (satya, śama, titikṣā and so on), the trance of the yogīs and the monistic bliss of Brahman all seem wonderful for men."
There are different types of perfections known as siddhi-vraja, and also the perfections of achieving brahminical qualifications, yogic trance and merging into the Supreme. All these material perfections are certainly very attractive for a mundane person, but their brilliance exists only as long as one does not take to devotional service. Devotional service can control the Supreme Personality of Godhead, who is the supreme controller of all universal affairs. The five rasas (mellows) in the transcendental world are practiced by the inhabitants of Goloka Vṛndāvana in neutrality, servitorship, friendship, parental affection and conjugal love. All these please the Lord so much that He is controlled by the devotees. For instance, mother Yaśodā was so advanced in devotional service that Kṛṣṇa agreed to be controlled by her stick. In other words, the five principal mellows are so great and glorious that they are able to control the Supreme Personality of Godhead. In the material world, however, the so-called siddhis, or perfections, manifest their brightness only as long as one is not interested in devotional service. In other words, the perfection of the karmīs, jñānīs, yogīs and others remains attractive only as long as one does not come to the point of devotional service, which is so great and significant that it can control the supreme controller, Kṛṣṇa.
(Sri Caitanya Caritamrta------2:19:165-------translation and purport).

"Beyond the region of ignorance (the material cosmic manifestation) lies the realm of Siddhaloka. The Siddhas reside there, absorbed in the bliss of Brahman. Demons killed by the Lord also attain that realm."
Tamas means darkness. The material world is dark, and beyond the material world is light. In other words, after passing through the entire material atmosphere, one can come to the luminous spiritual sky, whose impersonal effulgence is known as Siddhaloka. Māyāvādī philosophers who aspire to merge with the body of the Supreme Personality of Godhead, as well as demoniac persons who are killed by Kṛṣṇa, such as Kaṁsa and Śiśupāla, enter that Brahman effulgence. Yogīs who attain oneness through meditation according to the Patañjali yoga system also reach Siddhaloka. This is a verse from the Brahmāṇḍa Purāṇa.
(Sri Caitanya Caritamrta------1:5:39-------translation and purport).

What is the particular attraction that makes the Supreme Lord enthusiastic to accept devotional service, and what is the nature of such service? The Vedic scriptures inform us that the Supreme Personality of Godhead, the Absolute Truth, is self-sufficient, and that māyā, nescience, can never influence Him at all. Therefore the potency that overcomes the Supreme must be purely spiritual. Such a potency cannot be anything of the material manifestation. The bliss enjoyed by the Supreme Personality of Godhead cannot be of material composition, like the impersonalist conception of the bliss of Brahman. Devotional service is reciprocation between two, and therefore it cannot be located simply within one's self. Therefore the bliss of self-realization, brahmānanda, cannot be equated with devotional service.
The Supreme Personality of Godhead has three kinds of internal potency, namely the hlādinī-śakti, or pleasure potency, the sandhinī-śakti, or existential potency, and the samvit-śakti, or cognitive potency. In the Viṣṇu Purāṇa (1.12.69) the Lord is addressed as follows: "O Lord, You are the support of everything. The three attributes hlādinī, sandhinī and samvit exist in You as one spiritual energy. But the material modes, which cause happiness, misery and mixtures of the two, do not exist in You, for You have no material qualities."
(Sri Caitanya Caritamrta------1:4:60-------purport).

The destination of transcendence is to become the personal associate of the Personality of Godhead, who is known as adhokṣaja, He who is beyond the reach of the senses. The renouncers of the world, the sannyāsīs, give up all worldly connections, namely, family, wife, children, friends, home, wealth—everything—to attain the transcendental bliss of Brahman happiness. But adhokṣaja happiness is beyond Brahman happiness. The empiric philosophers enjoy a transcendental quality of bliss by philosophical speculation on the Supreme Truth, but beyond that pleasure is the pleasure enjoyed by Brahman in His eternal form of the Personality of Godhead. Brahman bliss is enjoyed by living entities after liberation from material bondage. But Parabrahman, the Personality of Godhead, enjoys eternally a bliss of His own potency, which is called the hlādinī potency. The empiric philosopher who studies Brahman by negation of the external features has not yet learned the quality of the hlādinī potency of Brahman. Out of many potencies of the Omnipotent, there are three features of His internal potency—namely saṁvit, sandhinī and hlādinī. And in spite of their strict adherence to the principles of yama, niyama, āsana, dhyāna, dhāraṇā and prāṇāyāma, the great yogīs and jñānīs are unable to enter into the internal potency of the Lord. This internal potency is, however, easily realized by the devotees of the Lord by dint of devotional service. Yuyudhāna achieved this stage of life, just as he achieved expert knowledge in military science from Arjuna. Thus his life was successful to the fullest extent from both the material and spiritual angles of vision. That is the way of devotional service to the Lord.
(Srimad Bhagavatam------3:1:31------purport).