A great game, until you go online.
Creating your own tracks is easy to do, with a simple to use editor, that can be used by anyone to create almost anything, it can take a lot of time to do good, complex tracks, but its worth it when you get to drive on it.
There are also hundreds of parts to use on your character and kart also.
Unfortunately then we get to the online side of the game. A lot of games have this trouble when they first start off. But modnation Racers suffers from constant server failures, after almost every race i get disconnected from the race server, and worst of all, the game thinks its ME quitting the race, it even has a stat in the stats window saying how many times you have been DC'd and it says 1 for me, even though its more like 5 or 6. Sometimes you can finish a race successfully, however its very annoying when you win a race and it marks it down as a loss and says you forfeited the race.
The publishing and downloading of content works fine however. Though i have noticed it is very unlikely people will play or use your content unless you advertise it or host a game and select the track as one of your own. Unlike LBP were almost every level is played without the need of advertising. Unfortunately this means in the online game lobby, known as the 'ModSpot' you will sometimes get spammers constantly advertising their mods, i believe the 'ModSpot' player limit is 24, so you shouldn't get too many chat messages arriving.
Really its a good game, the drivings nice, the creation tools are nice, the online is great when it works, its just trying to get it working.
Another small note is that it appears the game does have some hackers or glitches, as in the online hot lap mode, you people are posting ridiculously low times, such as 17 seconds around a track that usually takes about 50 seconds minimum.
This review is also posted on dooyoo.co.uk under the name of Jenraux.