I participated in two such studies once upon a time, but they were closely related. When my wisdom teeth started acting up, I didn’t want to cough up the money to have them removed, so I volunteered for two studies (one for left side of teeth, one for right) on pain medication following wisdom tooth removal. I wasn’t really looking to make money, so what they paid me I used to afford anesthesia during the procedure.
The study boiled down to getting your wisdom teeth removed and then receiving one of several dosage levels of the test medication, a dosage of the control medication, or a placebo. Then there were a number of blood draws at specified time intervals and self-reporting your pain level at that time; a catheter was left in the arm so you didn’t have to get stuck every time. Measurements were initially taken frequently, then tapered off; something like every 15 minutes the first hour, then every 20 minutes, etc. If the pain became too much to bear, you could ‘opt out’ and receive a regular painkiller; it may have been the same as the control med, but basically something known to work.
In between measurements, you essentially hanged out in their waiting area, watching TV, reading a book, whatever. Soft foods and drinks with straws were provided as well as blankets, pillows, and recliners to relax in. Most people seemed fine with the whole thing, at least as fine (as you can be after having your wisdom teeth out), chilling in their chairs and trying to be as unmiserable as possible. Naturally, though, there were some who WERE miserable, as you can imagine for those unlucky enough to get a placebo or low dosage of the pain med. One of the studies was a single-day, 12-hour thing; the other was an overnight stay in dorm-style beds. One or both of them required a short follow-up visit one week later for another blood draw.
All in all, it wasn’t a bad experience for me, but not necessarily something I would recommend for everyone. Occasional difficulty with blood draws and the occasional person in a great deal of pain were the only downsides I saw. I’m not scared of needles or nurses or such and not that sensitive to slight discomfort, so I didn’t have any problems. They tell you everything you need to know about the study up front for ethical reasons, so you know everything that is going to happen before participating in the study. All that said, my studies were relatively short, simple, and low-paying (about $300 total for both, I think it was); some of the other studies I’ve seen out there have longer stays, perhaps more discomfort, but of course higher levels of compensation, as well.
Hope that gives you some idea what it’s like (or can be like, YMMV).