Here is a video that shows a Japanese guy, from Japan, that has only trained with airsoft pistols and airsoft MSRs for 3 years. He has become extremely facile with these "guns." He was brought to the U.S. to try some real pistols and rifles.
Check out how fast he adapts to real calibers. On the rifle, he does really amazing. Most impressive is his malfunction handling. His muscle memory from using airsoft appears to have translated perfectly to standard firearms.
I'm a big believer in this, and I also practice with my airsoft rifle and Glock 19 replica. Like Will said, all the fire controls and weapon manipulation is the same as a real gun. As well as working on your transitions from Target to Target and between weapons as well.
It's a great way to train if you don't have time to get to the range. It's also a good tool to teach younger shooters and to let them get comfortable.