Amazing if you like speed running. Probably quite average if you don't.
If you're the kind of person who will just play the game once through and then not play it again - this is NOT for you. Only took around 1.5 hours to complete it first time for me. However, I've spend tens of hours playing since then, trying to get some great scores on the leaderboards. There's still a lot of time left for me on this game as well, definitely been worth the £10.
In some ways, it reminds me a lot of Mirror's Edge (now bear with me here...). Kind of boring if you just do the main story mode, but ridiculously fun when you start doing speed runs and time trials. Maybe it's just me who loves that kind of thing, but I definitely would recommend giving it a try.
Don't get put off by the fact that it's not like the original Sonic games. It felt very strange for me at first, but it doesn't take long to realise that the new controls and physics are perfectly fine when you get used to them.