DD outgrew her seat at 4 months due to her height. I was a bit surprised, because I thought I'd use the infant seat longer, but hey, it is what it is!
I really enjoyed the infant seat for when we were at church during the winter months. It was great to get her in the seat and throw the cover over her, and she'd be all nice and warm going into the cold car. (We always ran late, so no time to warm up the car!) Plus, she had difficultly with taking a nap, so when we were at our wits end, we would take her for a car ride and she'd be out in no time. It was great to just bring her inside without waking her up.
I'm really pleased with it now since I'm pregnant. I already have the infant seat for the new baby, just have to buy a convertible when the time comes.