The 10/22 is a great little rifle.
If you are looking to practice with something that has similar feel and controls as the ar-15 or m4 you already own, but shoots cheaper 22lr ammo, Id go with the M&P.
If you just want a good shooting, reliable, semi-auto 22lr that is easily modified/accessorized/customized I'd go with the 10/22 in a heartbeat.
If you already have the "guts" of a 10/22 a couple minutes at home with a screwdriver and an allen wrench will allow you to change stocks, barrels, trigger assemblies, etc. They are a great "tinkerers" gun for the guy that loves experimenting with different parts but doesn't own a machine shop.
Did I mention they can be great shooters?
Here are sixteen 5 shot groups at 25 yards with mine.
It likes the cheaper Wolf over Eley....go figure