Adfree improved my Angry Birds play on my sucky Spice to be a bit more tolerable, especially with Seasons.
Looking at some remaining ads I found that rovio1.appads.com also needs to be included in the hosts file, so it was not entirely perfect in blocking its ads. I added it manually to my hosts and the last bits of ads were gone.
In looking for this I also found that Angry Birds uses Burstly for its ads, which spews enough crap to plug up my logcat in a single run.
The good side is I only need to run it every so often to update the hosts file, then I can close it and leave it like that. Its built-in web server is entirely optional.
All in all, this is a must-have!