I've been using it for 5 weeks. So far, I love it. I like the app a lot--and it keeps me on track to see the numbers added up for me. I enter everything as I eat it (or sometimes the plan the night before).
Other people like LoseIt! which they say has better graphs. (MyFitnessPal doesn't have that great of a web interface even though its mobile app is good)
I think if you're calorie counting, using an app such as MyFitnessPal is really beneficial. As long as you enter everything you eat--no exceptions--you will have a real picture of what you're eating and what your calorie deficit is.
Common view is that MyFitnessPal underestimates net calorie goal and overestimates calorie burn for activities. That might be true.
Either way--log your food. Log your beverages. Log everything. It really makes a difference. (I'm told that last sentence is supported by studies, but I haven't looked up any primary sources.)
Goal: Back to where I was!