I play Gears 2 pretty frequently (multiplayer) and while I'm not in love with the combustible map-pack, I don't have any issues with it really.  Gold Rush is (IMO) a really good Annex/King of the Hill map as is Flood.  Feul Station is a good horde map, though otherwise I could do without it.

If the price dropped on the combustibles, I think you might like some it has to offer.

Unless its for L4D then no,Or R2,but then again both get FREE map packs :D

I'd get XBLA games, but maybe that's because I'm not huge into online multiplayer.

If that's your bread and butter then get them, otherwise I recommend Braid and Geo Wars 2 since you already said Castle Crashers didn't do anything for you. Also, Portal: Still Alive if you haven't already played it elsewhere.

I hate having to pay for something that should be free or included in the game, ie: map packs. That being said I would get the map packs if you play online a lot