Originally Posted by Eeyore
Brian put it best:
If you want it to look cool get an open bolt
If you want it to be reliable get a closed bolt.
I quite disagree with that. There's a big "if" in Bryan's assessment on the AWSS ("close-bolt") system. Yes it's more reliable from a performance's perspective IF you don't modify it with a NPAS system. Bryan has the advantage of being able to use his guns on his own field at 400+ fps wich, unless you play with his gang, you won't be able to do at most other places/fields.
My experience is with the original WE PDW converted into the open-bolt system. Originally it was shooting a very (too) healthy 435-445 fps wich is too much for any fields I play at. I thought about putting the RA-Tech NPAS on it and that's where all the bad experiences with this system happen. It's no secret, there's threads on every airsoft forums about that. Switching to the open-bolt dropped the fps to a more acceptable 345-365 fps (same performance on the stock PDW open-bolt), wich indeed means the system is less efficient than the original but in this case, to my advantage + you get the improved hop-up and less stress on the receiver, wich is less of a factor for the M4 or PDW than the SCAR. If you still need a NPAS to be installed on the open-bolt, that version is much simpler and reliable.
And you look cool.
*EDIT: If it's the M4 you want tho, there's no open-bolt for it yet...