TL;DR for this review: I recommend it, it was helpful for both allergies and stress. BRING READING MATERIAL. Wear the booties. After you turn into the center, it's down a level.
A friend recommended halotherapy when I mentioned my allergies had gotten really bad, on top of my stress levels getting out of control. I was really skeptical, but this friend is also usually very skeptical of things like this, and she loved her experience. I made an appointment at Austin Salt Cave and brought another friend. We had a little trouble finding it, as we were unfamiliar with the shopping center. Note that you have to drive down past the first place you could turn into.
The place is well-appointed, with everything you'd need to have a good experience, including lockers, towels, and reading lights. Which brings me to my first point: it's NOT recommended that you bring electronic devices into the salt cave, so either plan to meditate, nap, or bring reading material with you. The only real negative about my experience was that I wished I'd known this ahead of time so that I'd have brought a book to read. Otherwise, being in the room was pleasant, with soothing music playing, zero gravity chairs, and lots of pink Himalayan salt. A special device grinds pharmaceutical-grade salt down to a fine powder and distributes it through the room. You'll find a fine layer of this salt on everything at the end of your session (so I'd recommend wearing the booties provided, or you'll have salty socks/feet).
There's an introductory video made by the owner to help you understand the background of the process, its potential benefits, and what the experience will be like. You really do need to arrive 15 minutes ahead of your scheduled time to fill out paperwork and watch this video for your first appointment, though I'd have preferred to just read the info ahead of time.
I hadn't been able to breathe through both nostrils all week... I was completely clear after my session. What a relief! The calming effect also helped my stress levels. Long after my session, I'm still breathing better. I'll definitely be back, and I would recommend it for those with trouble with allergies.