Once. I only took it once and I was going through some very difficult emotional times, so I'm really surprised I did OK. I scored 710Q, 550V and 5.0A. Keep in mind, English is my second language, so I was pretty darn happy to score in the 74 percent of all Gre-takers for my verbal section. I know without another year of preparation, I couldn't improve my verbal score, and I was OK with my math score already, so I figured "what's the point?" It's competitive enough. It's not a fun test overall. I don't mind the material, but the fact that it's timed just makes me feel uneasy.
I had a GRE class once a week (part of an internship) for 9 weeks, which only helped with math, because it was a good review of things you just don't remember (like geometry). I think for verbal, you have to start early (a year or so in advance) and vocab. words study mostly helped me. I also had Kaplan, Baron, and official ETS Gre preparation books and WordSmart CDs. I personally like Kaplan a lot. However, my score is pretty average, so I'm not sure my advice is that great =)