Played it some more last night (well, couldnt sleep, so I got up at 2am and played it till 4:30am

) and I've changed my mind... its not 'amazing' - its abso-frigging-lutely superb!!
Really dont know how people are not liking this. They've taken the cart racing genre and pushed it to a great place - not just the modding, but the on-track racing is brilliant.
I think the addition of shields is genius as this balances the race out completely. Now if you are out in front, you can stay there (its harder, but least you have some protection from attacks). You get just the right amount of notice of an incoming attack and assuming you have the boost points saved, can shield yourself and stay out in front.
Also good that this cannot be abused (I think one shielding completely depletes your boost).
All the races I've had have been close and exciting with lots of action and the other new addition (in the cart games I've played) is the sideswipe attack - again very well done but effective.
The general presentation is brilliant too with the TV Show styling overlays and commentary. Sure the cartoons are a bit cheesy, but its nicely done.
Downloading and uploading mods is a breeze and very quick which is good and its nice to have sooooo many unlockables, achievements and the shop tokens to work towards.
Talking of mods, I had a good play in each yesterday and made a character, an IndyCar style car and also a track. I uploaded the character and car and though I didnt complete the track, using the editor was so easy!
I made a replica of the layout of Laguna Seca in about 5 minutes and testing it out was pretty accurate to what I remember (did it from memory). Could spend hours putting in all the details on the trackside.
Its cool you can jump very quickly in to test out the track either alone, or with some CPU cars to see how it works and then jump straight back into edit mode.
To round off the night, I did some timetrials (Hot Laps) and this was well done too, with 2 ghosts on the track - my best and the world best.
All in all, I think this is an amazing package. Sure the load times are long, but generally once you get IN something (i.e. a race, or edit, create etc), then there are no load times, so its balanced out.
What they have crammed into ModNation is pretty impressive and I really hope this gets some decent recognition.
9/10 for me.