The good- intervals allow you to burn a lot of calories in a short amount of time and keep your metabolism elevated long after you finish doing them.
The bad- if you are training legs two or even three times per week, you can not do intervals more than once a week without overtraining. Let me rephrase that; you can but eventually it will lead to overtraining or at the very least slow down your strength gains. You can negate this slightly by keeping your leg training volume extremely low and doing your intervals on the same day as your weight training. You can't do five to eight sets of legs two or three days a week and 30 minutes of intervals on top of it. That's a dead end road.
You also have to remember to do your intervals on your training days and not on off days like you might do with other forms of cardio because that will lead to overtraining much quicker.
The ugly- if you choose sprinting as your form of interval training you could get hurt; it's an ugly truth that has to be faced. The thing that will lead to even more injuries is following faulty interval protocol advice. Normally it is recommended to do 30-60 second intervals when they are being performed on a stationary bike. A lot of people take these recommendations and apply them to sprinting. This is a huge mistage! Nobody can sprint for 30-60 seconds. Ok, not nobody; but most average people can't do it. World class athletes can sprint for that long, but not everyone else.
Don't believe me?
Go try it. Warm up thoroughly and try to sprint for 60 seconds straight. Let me know what happens. We have all seen the Olympics and how winded guys are after sprinting the 100 which happens to last all of ten seconds. Most of us have seen guys run the 40 and not be able to catch their breath for at least a few minutes afterwards. And that takes five seconds or less. Not only is sprinting for 30-60 seconds impossible for most people but it also greatly increases the risk of injury.
When you keep your sprint distances and times very short, you decrease the risk for injury because you never hit top speed and instead spend most of your time in the acceleration phase. This phase has the least potential for injury. For that reason, most people should be running 20-50 yard sprints. This keeps you at top speed for a very short period of time; usually little enough time to maintain form and not suffer an injury. When you run at top speed for too long the chance for a break down in form and thus an injury is greatly increased.
I would never recommend that a non athlete ever try to sprint for 30-60 seconds straight and you should never take that advice from anyone. It is faulty and dangerous. To further reduce your injury while sprinting, use adequate rest periods between sets. Also, running with a sled slows you down enough to avoid top speeds and makes sprinting much safer.
Bottom Line- Intervals are a great tool for getting ripped, however when your main goal is to get big and strong and just keep fat gain to a minimum, they should be used sparingly if at all. I would recommend sprints above intervals on a bike and even then I wouldn't do them in true interval fashion but more of a traditional speed workout with short sprints and adequate rest periods. This will still elevate your metabolism greatly and keep you lean. Just look at the physiques of Olympic sprinters for proof of this; that his how they train. Sprint, rest… no intervals.