Reach is actually pretty fun so far! Bloom still sucks, but people also apparently really suck at dealing with it. You can't spam the magnum across an invasion map and expect it to do much.
There is definitely at advantage with a controller, at least for me, although it's probably not as huge as it seems if you're more used to the controller. The auto-aim but especially the bullet magnetism seem a bit too strong, though.
I have one of them mice with too many buttons mapped to all the important things, but the controller still seems way more convenient for quickly going between jump/shoot/grenades/melee/armor ability. But that's probably just because I'm so used to the playing Halo with the Elite controller at this point.
Teammates can still be pretty brain dead though, which can make something complicated like invasion irritating. Hey man, maybe you should use the falcon, to, like, stop the core so we win, instead of fucking around with two guys on the other side of the map.