Very easy for achievement's and fun for gameplay
There are two quite short "campaings". In one campaign you are a T-Rex Dinosaur, running around small cities destroying everything in your path while trying to eat up Dorito vans. In the other Campaign you are the Dorito van trying to deliver Doritos around the city while a huge T-Rex trys to eat you in its path of destruction. In my opinion, they are very easy and very short.
There are six levels in each campaign to complete and while you progress to each level you will recieve an upgrade either for your van or the T-Rex (depending what campaign you play). They are helpful upgrades as some boost your speed while others make you stronger. Thinking about it now the upgrades make the game so much more easier (they aren't really necessary to be honest, but, I think it makes the game more fun).
As you progress through the levels, they become more "harder". If you are the T-Rex you will have another T-Rex in the city trying to eat all the vans before you. If you are the Van, you will have another van trying to complete the deliveries before you. Its still easy though... xD
Achievements on this game are more than easy. That's all I have to say about the subject of Achievement's xD
Multiplayer on this game is really fun in my opinion. It's easy to set up and you can just basically have lots of fun with it.
Overall, I really like this game. I would recommend everyone to try it. Its free so if you don't like it, its not like you have spent a lot of Microsoft Points and then have the thought of "What a waste of my Microsoft Points ARRGG!" Try the game out and if you don't like it just play it for the achievement's.