If you missed Rez the first time around then this is a great oppurtunity to experience it!
I bought this game for PS2 years ago, although the original was on Dreamcast. It blew my mind upon first play and I played it quite a bit trying to be awesome at every level. This was not an easy goal as it is actually a quite difficult game when you get into collecting and destroying everything. Like most games that are balanced correctly, beating is challenging but do-able whereas actually getting 100% completion ratings is hard.
This version of the game seems to have a bit more than the other releases. It has a few extra modes that actually just allow you to play through the game again...but, differently? To be honest, I played one of the modes and just found myself playing every level back to back without any stopping point in between. I looked into more of the modes and they seemed to be the same thing. To be honest, I didn't really explore them all because I had fun just playing the normal iteration of Rez.
Now, I'm glad that I bought this new version of Rez because it's on my 360 with me every where I go now and since that has pretty much become my main system I don't have to worry about having another system, controller, disc, and memory card with me to play Rez (whenever the urge arrises), however, reading back my last paragraph it doesn't really sound like the game was worth it.
So, while I'm glad I bought the game, it's probably really worth it if you are looking to pick up some extra achievement points or you have a nice big HD tv and want to see the Rez glory in HD. The game is fantastic and if you don't know about you shouldn't hesitate to pick it up. Compared to finding the older copies of the games, a $10 downoad is a steal. But, if you already own the game it's hard to say you should or shouldn't buy it. There are good reasons to own it, but it really doesn't offer up anything new worth buying if you've mastered the old one and are through with it.