sprinting has been shown to improve all kinds of health and fitness parameters. here's a recent commentary re a study that, again, supports the value of high intensity training: http://www.multibriefs.com/briefs/acsm/active1-25.htm
now, before i go on and explain the pros and cons, let me provide a personal disclaimer: i don't train like i used to but i do intervals (on a stationary bike) at least 3/wk, plus a couple days of long, slow - 30 mins, RPE of 7/10 - rides. while i do not like the sprints, i do like completing the workouts faster, breathing harder, sweating more, and feeling accomplished....even tho i have absolutely no competitive goals. it just works when time doesn't.
so, the pros: more effort, less time, more anaerobic benefits, more strength benefits, similar cardio fitness benefits but no benefits to Q, or cardiac output, which is a measure of HR x stroke volume, or blood pumped per beat. uh oh, you say! well, don't fret it. when your heart beats fast, it does not have as much time to fill up, and therefore cannot pump out as much blood per beat. plus, this study was brief - 6 wks - and not all that intense. so over time, maybe you will get higher Q's. but think of this: the other component of maximal aerobic capacity is arterio-venous difference, or a-vO2, which is that peripheral component mentioned in the study. basically, your body gets better at extracting O2 from the blood to supply the muscles' screaming needs when it starts running short of O2 and long on lactic acid. in other words, the heart gets stronger, and the muscles get more efficient = improved aerobic capacity too.
and don't forget the anthropometric benefits - loss of body fat and waist circumference. in 6 weeks!!!!
now the cons: potentially dangerous if you have a bad heart. but sprints are relative - to your fitness level. so anything higher and harder than what you are accustomed to is a sprint. dangerous, potentially, but like any exercise program, start off slowly, allow about 6-8 wks to acclimate overall with slow, easy efforts, then gradually introduce sprints in short efforts with longer recoveries, as in this study: 30 sec/4 mins.
another con: more muscle soreness, esp if you run or stairmaster. but cycling will leave your muscles tired but not too sore, and the risk of joint injury is slim. (spinning classes, however, differ. the rides are hard and long, and these can make it more dangerous overall, and increase the risk of joint issues.)
another con, the biggest of all as far as i'm concerned, is that you have to psyche up to do them. it may be hard enough to gear up to go to the gym after a hard day at work, then harder still to gear up to do over-85% efforts, but if you do, if you can, the benefits keep rolling in. so, my advice: buck up, start with two days of 5 sprints for 30 seconds with a couple mins rest between, and see if, after a few weeks, they are not as threatening....and maybe even somewhat inviting. why?
because you'll now have more time to lift weights, silly:)