Last week I got the retail version of ModNation Racers. If you have previously read my ModNation Racers Beta Impressions, there isn't really alot more I can add (if you haven't read those Impressions, go read them now. I'll wait for you).
All I can say is, apart from the long loading times, there aren't alot of negatives for this title. If you like Mario Kart-ish racing games, and character customisation, then this is the title for you. Alot of the fun is looking online to find Characters, Karts and tracks created by other users. Some of the content out there is truly creative, much more than my effort Scottio (pictured above). I did make a Homer Simpson from scratch, and it only took me about 15 minutes to complete.
If you have a PS3, I highly recommend you give ModNation Racers a try. It is the perfect mix of Mario Kart, and Little Big Planet.
EDIT: Oh... and if you are playing ModNation Racers, go and download my creations =)