Played through the new stuff last night, it's not great, but it does add the big gun and the generator stuff, which modders can use in their own custom stuff now, which is cool.
Personally, I've been playing L4D for 10-20 hours a week since it came out, and have played it a lot more since the custom stuff started coming out. It's a game for me that almost relaxes me after a crappy day at work or whatever. Plus, I have a lot of friends on Steam to play with, and we all have mics, so we spend a lot of time just bullshitting around for an hour or so at a time.
If you're looking for some seriously awesome, lenghty L4D content, get the Dead City user made campaign. Its very polished and really good, with some wide open levels and just on normal it'll take you about 90 minutes to get through.