I do this every couple of months at a spa here in Austin. The tanks they have are big enough for me to stand in at 6'2 and not touch any walls at full stretch. I always go for a one hour float. Basically it is exactly like what you've probably read; you get in the tank and float and after a couple minutes your senses should be neutralized. Once the brain starts to wander free things can get pretty weird. I've had a different experience with each float, ranging from some interesting visual and auditory hallucinations to feeling like I might have fallen asleep in there/lucid dreaming type of stuff. Common denominator is I always come out feeling amazing and super relaxed. I've never had a massage or anything but I imagine the feeling of relaxation is similar. I also usually feel like my senses are heightened (brighter colors, etc.) for a day or two after. It's usually kind of pricey for a good spa, otherwise I'd probably go a couple times a month.
If you're going to do it I definitely recommend going into it with a plan to do more than one float. Visit the place you plan on going, check out the equipment, ask the staff whatever questions you have. The first time, you'll probably spend at least half the time adjusting to the experience and acclimating yourself to what's going on. I have history with anxiety and panic attacks and I was pretty damn nervous on my first trip -- the whole idea of getting in a tank in perfect dark/silence, breathing hot, humid air and not really knowing what exactly is going to happen can be kind of daunting. It can also be surprisingly tricky to figure out comfortable body position in the tank. Figuring out what to do with your arms/reduce stress on your neck while floating takes a little while. The spa here was really helpful and informative prior to getting in which helped reduce the nervousness but it still took about 30 minutes to really get comfortable in my first float. Second time around it only took a couple minutes and I was able to get more out of the hour.
