I've done them since I was a kid, way before I knew what they were (I guess I just thought it was cool that you could flex down there. I know, I'm weird)
You've already noticed one benefit of them, that you can grip your boyfriend. The stronger a woman's pubococcyl (sp?) muscles, the more she will respond to g-spot stimulation. So, if you do lots of them and it's really in shape, sex can feel better. I think it can probably contribute to strength of orgasms, too. Mine can last up to two minutes, with a sudden climax and then kinda rhythmic contractions. Do any other women have this?
I pretty much do them whenever I get bored. After awhile they feel good, but you probably want to start out slow and build up so that you don't get muscle soreness. After about a month of doing them, see if your boyfriend using his fingers on your g-spot feels better or more intense.
Kegel excercises will also be helpful in the long run, especially when you're older and your body begins to wear out a little. They'll help you so that you don't wet yourself, which in my humble opinion is a big plus!
edit: as for the masturbation, I find that it doesn't diminish my sex drive, but it can increase how long it takes for me to orgasm. My sex drive varies a little with my cycle, but not much. Stress and emotional distress will lower it more than anything else. If you're not masturbating to orgasm, I could definitely see how it would raise your desire, especially if that boyfriend or hubby is on his way home.