I've been, more than 10 years ago though.
Agree with stay the night - you want to be in there very early. Especially if you want to see the entrance not full of people (you have to walk through the Siq alleyway into the place - plenty of arabs offering horse rides for a price) We stayed in a small hostel but I understand that there are wall to wall posh hotels now.
The 'buildings' are stunning from the outside but the insides are basically hewn out rough stone - and very dark.
I don't think you could do it in a half day unless you only did the main tourist stuff. My general rule of thumb is that no matter how stunning the place you go the majority of tourists wont do more than 500m from the entrance/bus. We were there all day and didn't dawdle by any means. Get up to the high places - I assume there will be ropes or rails now on some of the more entertaining routes.
Actually, just looked it up on the webby, there are now museums and a visitor centre! What is the world coming to?
If you're in the area Jerash is an interesting site - Lots of good Roman and older stuff across Jordan and Syria as well as the crusader stuff.