For me, the gestures are a great idea, but have problems in practice.
Most notably, you have no idea what apps are open, or in what order, so you just have to keep swiping to find them. So you end up swiping across paused games, sites you'd viewed ages ago etc. in order to find the app you're looking for. It'd be much better if the multi-tasking bar (or something like it) appeared while you were swiping so you could see where in the 'list' the app you're looking for is.
Also, if you're swiping across any apps which 'fix' the orientation, it gets weird. If you're in portrait mode, and swipe to an app which is 'fixed' in landscape, you then have to swipe up/down to get to the next app, swiping right/left will bring up the multi-tasking bar! Even weirder, with some orientations (for instance if the app is 'landscape right' but you're holding the iPad 'landscape left') you might have to swipe left to go right and right to go left!