I have Real Guitar. I like the fact that it has its own player and isn't reliant on Kontakt, Halion or any other player to work. I like that it restructures my piano chords to something that is playable on the guitar. I like that the user interface is nicely tailored to what it does -- i.e. making a keyboard sound like a guitar. (Much like NI's Akoustic Piano UI is nicely tailored to all the things you might want to control about an acoustic piano -- whether the lid is open or closed, how much sympathetic vibration you get from the strings, etc.)
Real Guitar has a lot of options like fret noises and stuff -- the defaults seem a bit much of this for my taste. I usually turn them off. But it could be nice if you want to do some really dirty sounding blues or jazz where capturing every twang and buzz of the strings gives the piece its character.
The samples are okay. They are a couple of years old, so a newer sample library may offer higher quality by now. But Real Guitar certainly is good enough to fool you so that you wouldn't know it was played on a keyboard.
I wish it had more different types of guitars. For example, Garritan JABB has one that goes below the lowest note (E) on a standard guitar. An acoustic bass guitar would be nice, too, as would similar instruments like the banjo and mandolin. But what they give you is pretty decent and certainly worth the money. Most importantly it never gives me a problem (no crashing or other weirdness that I see in libraries dependent on third party sample players.)