A lot of commenters have talked about the pros of this routine, and there are many. Few interruptions, working while your mind is fresh, working within a regimen, etc. However, if you don't have an equivalent amount of discipline on at night to get enough rest, you will quickly burn yourself out. I did this routine for a lot of the second half of 2011, and since I'm a night owl by nature (going to bed at 12 midnight…
So although my sleep time was the same (I was still going to bed and waking up at the same time as before), by taking out my gym routine, I was actually feeling more tired, down and otherwise less alert. So I've been playing with alternating work/gym during the weekday mornings.