Hmm...I've got about 4 hours on my game now and I have mixed, but mostly positive, feelings. I like...
-being able to upgrade vehicles
-the diversity of the vehicles
-how challenging the races get less than half-way into the
game...they make you fight for your win!
-the new weapon upgrades, they're very well balanced because they can be difficult to put together
-the open-endedness of the tracks, and plenty to be had for completionists should I ever be feeling maschostic
-the stylus-controlled balloon popping is actually kind of fun, if not on the easy side (unless going for gold). It's nice to see an attempt at something different.
I dislike...
-how many

ing coins it takes to upgrade vehicles, because some of the later races are downright impossible without the upgrades
-the new boost system for the beginning of races. I end up having to use my finger on the touch screen because it's rather difficult to go from stylus to instantly holding down the acceleration button, and I can still never get a huge boost in the plane or cart. I'm afraid this will make things a lot harder near the end of the game.
-since you control where you're looking on the carpet rides, sometimes I'll miss a balloon or a coin and end up dragging the camera to an undesirable angle on accident. It blows and can ruin an otherwise great run.
-the complete omission of silver coins kind of sucks. why not leave them in, but just
add the carpet rides?? Adding content w/o removing old favorites would be much preferable, since those challenges are some of the hardest (and most satisfying) I've ever completed.
-most of the characters are fairly generic/lame, but I came to race, so I don't care all that much.
-the new voices are completely unnecessary. Taj used to have a goofy Indian accent, now he sounds like a British guy who's mother knew a man from India when he was in the 3rd grade and he's trying his best to imitate him but has no idea how bad it is.
Overall... it doesn't deserve the consistently and surprisingly low scores it's received. I say about a 6.5-7 is fair, and definitely worth the $15 I paid for it for a little nostalgia. Plus, online racing (which I haven't tried yet) will help me get over any complaints about less racing in the single-player because of the silver coins. At least there's still trophy races!
Try it before you buy it if possible, but if you can get it for cheap like I did, then it's even less than a budget title and fairly solid despite my gripes.