I've recently purchased a WP7, initially the browser was horrible, now they've upgraded it to IE9 it's much better, but I've still noticed problems on some pages, most notable of which is the new version of the Facebook Touch site. It's much better on my Android and IOS devices than it is on WP7, I'm not sure whether thats IE9's fault or Facebooks for not testing it on those devices, but it looks worse and has a bunch of rendering errors.

So the varying opinions are probably both Mango vs older WP7 versions, and also certain websites not being tested on it (perhaps using mobile webkit specific stuff)