MagicPrefs works very well for me. I use it to boost the tracking speed past Apple's own preference pane and it adds additional gestures like three finger click to invoke Exposé.
I use BetterTouchTool and have no complaints. It's dead simple to set up either global or application specific gestures. I wouldn't use a Magic Mouse without it.
I've been using BetterTouchTool as well even before I had a MM and it works like a charm.
I bought a Magic Mouse because of the things BetterTouchTool allows me to do with it. That says a TON about both what the Magic Mouse is missing and how awesome BetterTouchTool is.
I use MagicPrefs as well and I like all of the features it offers.