The newer Celeron's really aren't that bad. I posted a SuperPi time from my mother's laptop (About a year old with a Celeron) and it's pretty high up there. For basic web browsing and such, they are more than adequate. Just realize I'm talking about current generation Celerons. The older ones were junk.
As for the Atom... Eh, it's really what you expect. It's not really meant to be a desktop replacement or anything. Mine struggles to play flash videos and some 720p videos. But I mostly got it for casual web surfing, typing stuff, and chatting. I also keep mine around 900MHz/1GHz to help prolong the battery life.