If you look at the list of acts you might think this game is not very long..wrong!

Once you complete each set of 6 acts (3 with original sonic, 3 with new style sonic) it opens up challenge acts - some of which are the length of entire levels, some are mini levels which are the same theme as the main act but different design. There are 90 challenge acts! On top of this there are battles with other characters (such as Shadow) and boss battles - these are to collect the chaos emeralds.

If you want to go for the platinum trophy you will need to collect 5 red rings from each act and get S rank on them all - the ring collecting is quite challenging and had me resorting to youtube for help.

I absolutely loved it and hope for another installment - my only negative comment would be that when you are working on 100% completion, act 1 (old sonic) is usually much easier for challenges and red rings than act 2 (new sonic) - when I am in one zone I would prefer to find the level of challenge the same.

If you are buying this for a very young player, although the boss stages aren't overly difficult - it could prove an obstacle as you can't progress further if you get stuck on a boss level (and there is no difficulty setting).