I do not like the fact that the bolt does not stay back when the magazine is empty. The difference felt when the bolt stops in the back position warns you that you have just run your magazine dry.
After you get used to it from firing enough rounds, you are eventually able to hear the bolt closing smoothly over an empty magazine instead of the shearing sound you hear when the bolt strips off another round from the mag. There is a noticeable difference.
mamelukesabre: You also need the F/A bolt carrier in addition to your lawfully NFA-registered F/A sear trigger pack, but you can have a decent gunsmith convert a semi-auto carrier for you.