I love the M2 Carbine so much that I couldn't get by with just one.
Having both a registered receiver M2 and a registered trigger pack, here are my thoughts...
Pro --- Ammo availability: I can find half a dozen deals on .30 Carbine in just a couple of minutes. Try doing that with 9mm, 45 ACP, 5.56 or 7.62 right now. I never thought I'd see the day when .30 Carbine was cheaper and more readily available than 9mm.
Pro --- Cost: The M2 Carbine is (or was) significantly cheaper than a lot of full auto rifles and submachineguns. At the time I bought my first M2, it was the best deal on the market.
Pro --- Fun as hell to shoot: My mom and cousin, both of whom are not really "gun people", love to shoot my M1 and M2s.
Pro and Con --- Durability: A registered trigger pack is hell for stout. I've heard that M2 receivers can crack rather easily.
Con --- Reliability: My M2s run like scalded cats, other people have reported problems with their M2s. I think most problems are magazine related.
Con --- Lack of Variety: The M2 is a one trick pony.
Con --- Magazines: Some of the 30 round magazines are overpriced and under-performing. The 15 rounders go way too quickly on full auto.
My advice...
For the budget minded shooter, go with a good USGI registered receiver M2. You'll save some coin by going this route.
For a hardcore addict, spend the extra coin and get a registered trigger pack.
In Liberty,
KristopherH