crunches are good but too much of anything isn't necessarily good- the body gets used to them so it cheats and has to work less at that motion over time- and it effectively becomes less of a workout. also, it all really boils down to proper form- i hate when i see guys in the gym clutching their heads in a death grip and making rapid half-motion crunches while jerking their heads forward with their hands- it accomplishes nothing and is dangerous. give me 50 good, slow, protracted crunches with one's gut sucked in and fingertips lightly touching the ears over 500 crappy crunches any day.
also, the abs are a complicated set of muscles- think of how many ways the torso can twist and bend! therefor the best way to get good abs is to hit them with VARIETY. i originally read that in here in a fitness article actually- and have found it to be very true. just take 10-15 minutes every day to do this sequence 3 times over: 20 leg lifts, 20 side crunches on each side, 15 lemon squeezes, 20 bicycles, and 20 crunches. if you wanna get fancy, hold a pike position (like a push up position but on your forearms, with a perfectly straight back and sucked in stomach) for 30 seconds afterwards.