Just as the title says really! I have a 3 month old, almost 4 months, I go to baby groups twice a week and all the babies around her age are in such fancy clothes! I always just stick her in a baby grow (or onesie as I call them) every day. I just find it so much easier and with this cold weather the build-in mittens are a god send 
I'm wondering if i should be a bit more experimental or keep using them, and what age people have stopped putting their babies in them?