The slick new Google Groups launched in October last year and available at groups-beta.google.com is now out of beta and moves to groups.google.com.
You'll find a cleaner homepage, search results that look more like web search results, topics that borrow a lot from Gmail's conversations. Each group has 100 MB storage space you can use to upload files or to collaboratively edit documents. Groups are much more customizable: you can change the colors and add welcome messages.
Now Google should focus on creating some filters for spam posts and a way to discover potentially interesting groups.