I LOVED Carters brand - they're very long. Gerber runs 2-3 sizes small imo. I didn't really like most of the stuff you find at Target or Wal-Mart, they all seemed short and wide to me.
We're still wearing Gerber 0-3 but they need to be retired. He finally fits in the Circo ones. DS was 7lb 7oz at birth and EVERYTHING we had was ginormous on him for some reason. I hadn't bought any gerber onesies because they run so small but I whipped out a pack of the newborn size that we got as a shower gift and that was all he wore for like the first week! I agree with PP that so many of them are too short/wide.
All of them have worked for me, but you generally need them 1-2 sizes larger than your LO would normally wear. I agree that Circo are great as are Gap.
Re: what onesies fit over CD?