Roller Coaster Tycoon 3I installed RCT3 yesterday and spent about three hours with it. How you respond to this game will probably depend on where you belong in the following groups.
Type A: Played Roller Coaster Tycoon, loved it, didn't get sick of it, couldn't wait for the sequel. Response: you will absolutely love this game. The graphics look fantastic, some of the new features (fireworks displays and the coaster cam, for example) are spiffy, and it's Roller Coaster Tycoon turned up to eleven.
Type B: Never played Roller Coaster Tycoon. You will also probably love this game. If you've never played it, the game is remarkably fresh, funny, and ingenius. Plus, you'll get your first exposure to the game with 3D graphics, which look wonderful.
Type C: Played Roller Coaster Tycoon and loved it--for a while. After thirty hours or so, suddenly realized that repositioning trash collectors to limit the puke accumulation near the Cosmic Vomitron ride was no longer refreshing. I'm not sure there's enough new here to keep you interested beyond the initial "Oh, wow!" effect of seeing the amusement parks in 3D.