I bought my D5100 in July and have really enjoyed it. Love the swivel LCD screen makes taking low level or high level shots much easier. When setting up the tripod I don't need to be able to get behind it completely to see what I'm taking. Great addition.
As for the flash commander - when I bought it I didn't realize what that would mean - now that I know I wish the D5100 had it - but I did buy an SB700 which as I understand it will act as the commander on the D5100 and can control a remote Nikon flash. I'm thinking that I'll either spring for a SB900/910 in the future or save a few hundred dollars and learn how to manage flashes manually and go with the optical slave route - or use flash triggers. But that is a little ways off as I'm just learning both the camera and the flash.
I really want to get past auto-everything which I tended to use on my old SLR (Nikon film) - the cost of developing discouraged experimenting now that I'm digital I can "ruin" lots of shots and the only cost is the time to hit Delete. Been concentrating on using the A setting for apature priority and controlling depth of field.