I know that TC can sometimes crash when caching large areas... although I think it's just my bad memory sticks I need to RMA. But, yeah, the sky is too fast, but the Trainz team is working on a fix for it.
Improved shadows, improved performance most of the time, undo bug fixed, CMP is little more stable (but still lots of bugs like before)... um, a lot more. There's a lot more under the hood than you think.
It has less built-in content than TRS2006, which could pose a problem, but if you have TRS2006 installed (or you bought Merscom's "Trainz Railwayz" which includes TRS2006 with TC), you can import all your content (built-in too) from it. Just do a forum such. :wave:
I'd say it's worth buying it.