I bought Railworks about a week ago and the only thing I can say against it is that it's highly addictive. I'm new to it, still learning the ropes, but I've built 5 miles of my own route and landscaped part of it and everything works OK except signals that I'm still experimenting with. The realism is good unless you get really close to anything, but that's the same with most 3D environements. To me Railworks is a lot more involving and a lot cheaper than creating a model railway in the spare bedroom!
The ready-to-run routes that come with Railworks are amazing - especially the Somerset & Dorset from Bath to Templecombe in the snow, for instance.
For me the frame rate and smoothness is good - a lot better than one might be led to believe by watching jerky YouTube videos of Railworks - it's not like that at all, not for me anyway.
Today we managed to save and transfer my route from one computer to another, which is nice to know.