Got into the Modnation beta, toyed around with it a bit last night and a fair amount today.

Initial impressions:
+ general tone of the game has a cool vibe, lots personality
+ tons of customization for your racer and car, despite a limited selection (most of it is locked out)
+ customization can go into exacting, granular detail, the size of noses, or stickers, orientation, colour, angle, etc...
+ creating a track is surprisingly easy, and you can make something that is readily playable (and fun) in less than ten minutes
+ just like your character or car, detailing your track is very...uhh...detailed. You can narrow the track, make it slant, created uphill climbs, underground passages and bridges. If you're creating road underneath an existing piece of track, the older parts will automatically morph appropriately into a bridged area. There's a nifty feature that also intelligently map out short cut routes on the track
+obstacles, power ups, and speed boosts are available
+ my favorite game feature included - in game screen cap!!! You can see more of my own pics from the beta here:
http://picasaweb.google.com/rey.ortega/ModnationRacer#
- no loops (at least I don't know how to make one yet)
- not much in the way of tutorials, documentation on how to use creator tools
- load times feel generally longer than they need to - likely due to beta code/not yet optimized
- framerate a little sketchy when in the hubworld online. I've been playing mostly offline
- have yet to try an online race (waited for about 6 minutes and exited out)
+/- customizing car/character is superficial, no benefit for using say, different engines or materials
+/- music is somewhat forgettable