It's a great option, but it has some drawbacks for me at least:
1) The lower isn't a real lower. Proprietary. It's not an AR, it's AR-ish.
2) Proprietary mags. The standard 22 mags will work with all known ciener bolts and rifles like the sig 522.
3) If you use a conversion bolt, you get to use/train with your normal AR.
4) No flash hinder/compensator. I know it serves no purpose with a .22 but it just looks weird without one. I dislike anything that resembles ban-era =)
The only real advantage (for me) outside of cost for the m&p15-22 was a working bolt hold open. Seems that's going to change soon.