I'll print out and read errata for books I have: but I don't directly implement it all the time. To be honest, it's hard enough running a session with a few books open and my adventure notes: also having to reference additional sheets would drive me nuts.
If something overtly doesn't work in my game I would check for errata and implement that; something that's much talked about for being particularly broken, I would consult the errata before thinking about using. (i.e. a couple of the mosnters in Monster Manual III) I don't really write inside my books, but I do sometimes leave errata lodged inside if I think I might want to check it mid-game.
So, I like having it for when it's a real problem - but most of the time I struggle on.