And lets not forget about GTR 3 by Waves. Although I havent played with it at all, I hear its pretty killer! It has also been said that its better sounding than Guitar Rig and Revalver although I cannot confirm this personally.
Its all just heresay really!
From my point of view though I am thankful that 2008 has been the year of guitar emulators. I've been a drummer all my life and recently had to stop playing due to medical reasons. So I decided to take up guitar and after a few months of saving I eventually bought my first ever electric guitar.
However, having spent all my money on the actual instrument I had no money left over to buy an amp. It was then that I heard of Revalver and downloaded the demo version. I was so impressed that I ended up buying it and Guitar Rig 3.
I am happy to say that I have not looked back and do not regret getting them. I know there are a lot of 'purists' who would not agree with me but with apps like Guitar Rig and Revalver I can play on amps that I would normally never be able to afford and its extremely simple and convenient.
I believe that they come extremely close to the real thing and for people like me its a godsend!