I was addicted to Diet Dr Pepper - I drank about a six pack a day for nearly 5 years. When I was skinny (~130-135 lbs), I did not gain weight drinking all that soda. However, when my metabolism slowed down, and I started gaining weight, I wondered about the effect the diet soda may be having on me - but I didn't stop drinking it. I liked it too much.
I decided to stop drinking the stuff last summer (August 2009), when I came to the conclusion that it just couldn't be good for me to drink so much soda all the time. I had been trying (rather unsuccessfully) to lose weight since early 2009, and decided to just cut it out all together and see what happened. Long story short, I didn't lose weight by eliminating the soda from my diet - but I did feel better. I can't really explain it, but I've not been as tired, I sleep better, etc. I have occasionally given in to temptation, but I would estimate that since August 2009 I've had a soda less than 10 times.
As for weight loss - I've been calorie counting since June 30, 2010, and have lost almost 20 lbs! Again, I'm not sure if the absence of diet soda has helped at all with my diet, but I'm not about to tempt fate and start drinking it again!