Well, since everyone is talking about it, I'll share my thoughts from playing for about a week now:
Positives:
+ The load times are much improved over MNR, more in the LBP zone. I didn't feel like I was ever waiting too long.
+ It has that wonderful LBP charm to it which I very much enjoy.
+ Battle matches are awesome. Seriously awesome.
+ HUGE variety in game modes makes it great. Isometric, motorstorm-RC esque races mixed in with egg throwing contests alongside all the traditional modes. Really cool.
+ Customisation levels are as high as I'd expect from a LBP game. I enjoyed the different things you could do with your kart, and the track creator is better than MNR.
Negatives:
- Too slow. I know you can tweak it, but the base game is just too slow.
- Framerate issues when there's a lot of stuff going on-screen (particularly in busily designed levels)
- Tutorials are too disconnected from the rest of the game. It's a problem I had with LBP as well. I don't want to have to sit through 40 minutes of tutorials and then try and remember all of it when I'm making my track.
Overall, it's a very solid beta though and I will be buying it at launch.