For android, generally much better. For iPhone, because it has been out for longer and are so many people developing for it, the really good iPhone apps are top notch and often better then WP7 apps (give WP7 time). But, conversely, with such a huge marketplace on the iPhone, there are 100K horrible apps.
I think in general the WP7 apps are really nice. And the OS is done so that it does not really effect the phone if an app messes up.
Until the next version of the OS, coming out in a few months though, the phones are really tied down as to what the apps are allowed to do; so you might find that a little odd at first. But the apps that are already out in beta that we know about for the new more open OS are amazing, and odds are pretty good by the end of the year everything will be golden.
The rate of the apps and developers coming to WP7 has been consistently faster and better then either iPhone or Android.