I know I was the one who requested this thread, but I'm not going to choose a score just yet as I don't think I've played it for long enough. Suffice to say that it is currently in the 8-10 range from my limited play test.
Great graphics - absolutly gorgeous IMO. Especially when you have lauched yourself 100ft into the air!
Good AI - you can see them shunting and barging each other at the start line and throughout the race (speaking of which, its a shame you can't punch people in this

), they don't land every jump (I've won a few races because the leader hasn't landed the final jump and crashed

). There doesn't seem to be too much elastic band syndrome either, and there isn't a noticeable set line that they all follow like you notice in some racing games
Sound - the soundtrack in this game is pretty good as it is the kind of music i listen to anyway and fits the vibe of the game nicely. Ambient sounds all seem decent enough.
Gameplay - this is where the game shines. There is nothing else out on the 360 (apart from maybe baja?) which allows you the insane amout of air you can pull in this game. The way the boost and combo types you can string together are combined is a nice touch giving you the choice of being able to boost for a little bit but not pulling any big tricks or saving your boost on the initial laps so you can get the big tricks, big air and bit boost to eventually get the star boost. The tracks I have played so far have been great. Starting off with ones which you think are pretty extreme and immediately upping the jump size almost from the first race after the tutorial depending on what track you choose. The ATVs handle as one would expect and the different terrain surfaces really are noticeably different.
The upgrading system seems a little bit over the top and there are far more upgrades and options than one would expect from an ATV game which is good but can get in the way of you wanting to race. Although I assume it becomes more important customising your ride to suit the next track as you get further into the game.
The game isn't too difficult and there is a certain amount of leeway landing tricks and you aren't penalised to heavily if you crash. A second or to out of the race and (I think) a little loss of boost. The tricks themselves look cool and are easy enough to pull off, its a shame though that you aren't drip fed them as you progress through the game as it looks like you have all the tricks available from the start. Each progressive level of trick is assigned to 3 buttons on the face of the controller. To do a trick you simply push the trick button you currently have access to and a direction on the analogue stick, so not too difficult and you can string multiple trick combos together in mid flight for more boost.
Another great touch is that most races (especially the longer ones) have multiple paths you can take. And each multiple path then has further multiple paths branching off it so there is a lot of replay on the tracks finding the fastest lines or the best lines for getting boost etc. It also means that every race/lap feels completely different so it doesn't feel like you are racing exactly the same track if you decide to replay them a few times to get a decent position.
So far I am most impressed with this game as I hadn't really heard anything about it. I'm hoping multiplayer will be good as I haven't ventured online with it just yet as the single player is really good. But the multiplayer has the potential for being awesome.