I hate to give you a lame answer, but the truth is YMMV (Your Mileage May Vary).

Some people have a problem with aspartame, some don't. Some people begin to ravenously crave sweets, some don't. Some, like me, get headaches from too much aspartame. I think it has caused some people stalls, but I don't think I've ever heard of anyone actually gaining weight from it.

Here are a couple threads you might find interesting:
Okay, so what's REALLY wrong with Diet Coke?
Aspartame Poll

Basically it's an individual thing. But... Diet Rite made with Splenda is quite good. Check it out. It comes in Cola, Black Cherry, White Grape, Red Raspberry, Tangerine, and Kiwi Strawberry.

You really have to figure out what works for you.