I personally use a mic'd amp for all my final tracks, but when throwing down a scratch track to write off of, i often go into a U5 and then directly into Pro Tools. Throw amp farm on it and you get yourself a decent sounding track.
When i do my final tracking i always run a direct signal through the U5 onto it's own track, in addition to my two or three mic'd tracks. I almost never use it, but i have brought it into the mix before to where it definitely colored the overall guitar sound to what i was looking for.
The U5 is more productive with a bass as you most likely will record more direct in bass tracks than guitar. If you have another grand to spend, I'd invest the $500 towards an Avalon 737. That's a full tube channel that sounds great on alot of things.