Halo: Reach has had four map pack installments, each adding to the multiplayer experience, or so they say, but what do they specifically bring that's new to Reach?
The Noble map pack pack is the same affair that all big FPS games get, more maps, pay money, done. And the defiant map is no exception, but with the defiant maps from Halo:2 are added to give players a nostalgia kick. In general the maps are quite good, everyone will have their favourites and their worst maps, but that happens with every game with multiplayer access.
The Anniversary map pack is the largest map pack, bringing in 7 maps from the first ever halo game, re-done in HD and for multiplayer carnage, this map pack can be purchased on it's own, or if you purchase Halo:CE:AE you get a lovely little piece of card with a code to download them for free. This map pack adds new to features, like with the previous 2 map packs it gets it's own playlist, but gameplay is tweaked, such as the pistol is overpowered like in Halo:CE and armour abilities are gone.
I can't really say "GO AND BUY IT!" because it depends on your opinion of Halo: Reach, but if you do like Halo, give 'em a whirl.