I just purchased the new Smith and Wesson M&P 15-22.  The rifle seems well made and has a very similar feel to my Bushmaster AR-15.  The biggest difference I notice is that the rifle doesn't have the same weight as a true AR-15, but for a plinking rifle that is fine.
I mounted a scope on the rifle; needed to use high rings for my 40mm objective, and went to the range for a little fun.  After carefully loading a magazine, one round at a time per the owner's manual, I fired my first round and everything seemed to work fine.  I squeezed the trigger again but nothing happened.  I pulled the bolt back and found that the previously fired case failed to eject properly.  I pulled the charging hammer, cleared the spent case, and loaded another round.
I fired again but again the spent case failed to extract from the rifle. I unloaded the magazine and carefully filled the magazine with a different type of ammo; thinking the ammo might be the problem.  Still had the same problem.  A friend of mine who also bought a M&P 15-22 was shooting next to me so I grabbed his rifle and switched out the bolt assembly thinking that maybe the ejector wasn't functioning.
He fired ten rounds through his rifle with my bolt and experienced  no problems at all.  I was still experiencing the same failure to eject problem; even with his bolt.  I believe the problem is with the ejector inside the rifle failing to kick the spent case out.  I have called S&W and they are sending me the information I need to send the rifle in for repair.
After checking on the web this is not the first instance of this happening with this rifle.  I know that Smith and Wesson has an outstanding warranty on there rifles and that they will stand behind there product.  It just kinds sucks when you don't even get to shoot your brand new rifle before sending it back.  I think S&W needs to do a function test on each rifle before it leaves the factory to make sure that things like this aren't happening
 
Here is a YouTube video of someone else experiencing the same issues as me -
http://www.youtube.com/watch?v=-xTptp3_g2M
 
 Update - 
 
From the day I called Smith and Wesson it took 28 days to get the rifle back in my hands.  The bulk of this time was waiting for the shipping label and mail time; actual time at the repair facility was only 6 days.  I do appreciate the fact that when I received my rifle they had shipped a spare magazine back with it.  
I took the rifle out for a function test to make sure the problem was solved.  I fired 300 rounds of federal bulk .22LR through the gun without a single failure to eject or failure to fire.  The only problem that I see having in the future is trying to keep enough ammo around to go shooting.