We're switching sometime next week, when the one we ordered for my car gets in (silly me bought a 2 door car before getting pregnant, whoops!) For us, the reason for switching is that it has gotten to the point where a) I can't carry DD in the bucket style seat more than a few feet now because the combo is so heavy and b) she's tall enough that her shoulders are way up where the seat narrows and barely fit in, and there literally is NO slack left on the straps at all, we can't even have her in a thick sweater in the seat anymore. it's clearly no longer comfortable for her.
This weekend, we will probably also be looking for a convertible to go in DH's car, but his is much easier as he has a 4 door.