yes it does make you stronger for slow twich fiber muscles and your fast twich fiber muscles (at the same time). If you want to work on your fast twich fiber muscles (performing faster for your sport, improve sprinting time, and such)...isometric training is the way to go. I read articles on this at athleticquickness.com very true. example - if you hold a dumbbell straight out in front of you in one hand and a rubberband in the other hand, which will hit the floor first? the rubberband or dumbbell? The answer is the rubberband. If you bend a rubberband back and let go, it will quickly fly across the room.
isometric training - fast twich fiber muscles only/flexibility
weightraining/polymetrics/cardio/sprint work - is all slow twich fiber/fast twich fiber muscles working together, which takes a lot longer to perform faster. Its good for endurance, strength, and such.
Both are very important. I cant wait til I get the book I ordered at athleticquickness.com to start training with isometric bands for soccer and becoming a faster runner and such. I just really hope what was said on that site is correct and I do improve with this isometric training. Everything seems correct. Great examples about rubberband/weights make me believe that the site is perfectly accurate.
for weightraining - all I do is circuit training. I am not about building hypertrophy (getting bulk and big muscle), so circuit training is the best. I felt like crap the other way, but with circuit training my muscles are happy. and plus I'm not in the weightroom as long, so circuit training rocks =)