^NL (No Limits) simply blows RCT3 out of the water.
OK, you can't build a whole park, and you're only limited to one track per landscape, but it is so advanced that it makes it worth it.
Firstly there's the building. Unlike RCT3 there are, funnily enough, no limits. You can build whatever elements you like, and the simple wireframe editor makes it very easy. After the track is complete you also have control over the supports. NL basically works using sets of nodes. Track nodes have control points, allow you to change their position and shape, while support nodes allow you to join beams of varying sizes to other beams, or footer.
Secondly there's the physics. NL has incredible good physics included. You can get readouts of vertical, lateral and acceleration Gs, along with accurate speed readings.
Finally there's the graphics. The cars, track, and mechanisms are very well represented. The graphic are truly awesome.
There are also tons of other features. For example, there are advanced 3Ds object options, plenty of trees and an almost infinite possibility for landscaping.
It's hard to get to grips with, sure, I'm not going to lie, but once you have got used to it you have so much control that it becomes quite a powerful game.
The price of the game is truly outrageous. When I bought NL about a year and a half ago I paid just over £16 ($ to £ conversion). For a game of this quality that price is a steal. Definitely worth it.