I work 8:30-5:30, so going when I actually *want* to go isn't an option. If I could choose anytime, I'd go at about 2 p.m. because that's when I felt I had the most energy at the gym. I end up going around 8 p.m. and stay at the gym until about 9:15. I usually try to go earlier, around 7:15, but I have a tough time getting my arse out the door (for anything, not just the gym.. for some reason, that's just how I am!)

I agree- work out whenever you can fit it in. The "optimal" time could very well be different for everyone.