i'm using M3's in the back and I think they work very well if you position them correctly. Unless you have very properly places furniture, or a wall to wall mount them, don't forget that you will need to buy stands. This adds another $100 or so to the price. If you don't mind making that expense, go ahead and buy them.
I'm assuming if you watch mostly movies, the difference between M3 and M2 in that back would be slim. There's usually not much going on back there. DVD audio and simulated 5 or 6 channel audio, however, might require better speakers if you are very picky.