I think between 2001 and 2004 I went through sort of a freshman-sophomore-junior-senior sequence with RCT1, RCT2, Hyper Rails and No Limits respectively. I don't know if it's still around or not, but Hyper Rails is probably halfway between the RCTs and NL, because it has a lot of pre-fab sections you can add to a track as you're designing it, but you can put more variation into them than you can with the RCT programs. Only trouble was, it didn't have anywhere near the number of coaster types NL does!
The only beef I've had with NL is that every time there was another version, my frame rate dropped by about half, at least on my old computer. When I got a new one with a graphics card that could handle it, I realized that I could transfer track files from my old one to my new one, but not the program itself. I thought if I was going to have to buy it again, I'd wait until NL2 was out, but I don't imagine anybody knows when that's going to be! Finally broke down and got NL 1.8 last weekend.
I don't know if this is a new feature for 1.8 or if it was out earlier, but the "Tracking View" is almost more fun than the front seat view. It's like cameras at every segment of the ride, showing a train coming towards it and then panning and showing the train going off in the other direction. Sort of like they'd have at an Olympic skiing event, I guess. It also makes it look like it's going about twice as fast as it does on the on-ride view!
It looks like it also has a track smoothing feature (CTRL-G, I think), which used to be against the rules in the comps, but I wonder if that was when the "Node Smoother" was a separate program. I always assumed the rules meant you could only use whatever features came with NL when you bought it, but I'll have to get some clarification on that the next time I take part in a comp!