I've been doing 3 sets of 30 sit-ups every other day for a while. I was wondering what exactly are the benefit of sit-ups? Strengthens the stomach muscles?

are you doing sit ups every day?

Not every day, every other day, so about 3-4 times per week.

kills time if you're bored

can't think of anything else

I do ab crunches on the ab ball and I can tell that it's toning my ab muscles. If you want to see more dramatic results to weighted crunches.

So what is actually the best exercise for getting a 6 pack? Aside from losing that fat obviously.

Is that ab crunch machine at gyms any good?

The "in" thing is cunches and balls to work "core" muscles. Really the abs are a big stablizer for your body plus it gives you a flat stomach after you remove fat with diet.

Situps don't attack the abs that effectively.