I've been playing tennis for almost as long as I can remember. I think I started really when I was around 8, and I love it, but it has done negative things to my body as well as positive. I played it pretty much year round until this year, my first year in college... I don't think I've touched a raquet since June, which kind of breaks my heart in a way, but I know I'll still do it on a daily basis my entire life. That's just the kind of sport is is.
I was varsity/captain in high school and it was pretty intense competition, but I never got too competitive. I always just played for fun--I didn't really make it my life, however, it has always been a big part of my life. I was recruited to play Div III in college, but chose not to because I really just want to focus on my studies and live my life while I'm young, and I think tennis would just get in the way of that.
Obviously, it improves hand-eye coordination, endurance and agility, but I still can't run long distances without getting winded. I can go in sprints for hours and hours, but that's because, well, that's how tennis works. I also attribute that to smoking regularly for the past two years, but my game hasn't gone --that-- downhill--in many ways I did improve, I'm just not improving at the same rate if I hadn't picked up smoking... oh well. I suppose it also did tone my body, especially my arms, and to some extent my legs, but I think in order to get really great you have to do A LOT of muscle/weight training, which I did on occasion, but never with the intensity I should have, had I wanted to compete nationally... it's always just been fun for me, and I can't wait until it gets warmer out so I can play again.
The negatives, though, I got bad tendonitis in my wrists/hands my junior year in high school and almost had to have steroid injections, but I just stopped playing with such intensity and it has gone away; I still get a lot of soreness and have some joint problems, though. I've never had tennis elbow, but that will probably happen when I get older...