I'm currently going through this "excersise" of trying to create custom tracks for my kart series. It's not full of them BTW, but will include at least two.
Mt Asshole - I've named one of them. I wanted a really tough track for karts, but Mt. Aso as a template is very bumpy and jittery. The abuse your wrists take is brutal. I experimented with many settings and i've improved the shakiness, but it's still horrible.
I love the LAYOUT of some of the Mt. Aso tracks, and love the feeling of traveling through the mountains (something that I need for the series), but I may end up axing this course template entirely if I can't find some secret to getting karts not to shake while going around it.
Toscana - I've had more success with this one, as some of you have said. It stays relatively smooth compared to Mt. Asshole. Love the scenery too. You'll even find the occasionally guy taking a dump on the sidelines behind a bush!
Eifel - Best of the three IMO. It looks like a race track, and can feel like one too if you know what you're doing. The best "flow" of tracks that I've seen have been at this location.
Either way, all three can be good for something. Though, I feel they get a bad rap because people carelessly throw numbers at them and then say "see look what I made!".
The ones really good at making these courses should get involved with sharing them.
The_Revengel runs a "Trackshare" account that used to share tracks on a weekly basis. I think if interest picked up, he'd begin rotating tracks into it again. His tracks in particular are really good.
here is the thread for that "club" that isn't very active at the moment.
Tracksharing Thread