This is hands down the best superhero game that has been made to date, the freedom element works extremely well and the dark snowy setting looks visually fantastic.
There is so much to do outside the campaign, there are quite a lot of side missions - some are only one mission long and others go on for longer. I dont want to give anything away but one mission has you simply fighting some waves of enemies and then another has you trying to locate a killer by tracking his phonecalls across the city.
Then there is also all the riddler trophies and the riddles, there are hundreds of trophies set across the game and some can only be collected once you have the relevant gadgets like in Arkham Asylum.
Another reviewer mentioned you have to play as Catwoman, this is true however her missions aren't very long and how long you spend playing as her outside these missions is entirely up to you.
If you played Arkham Asylum you will find the controls and gadgets are pretty much the same but with a few small tweaks like the added ability to glide across the city.
As for Batman not being able to swim - the guy has hundred of pounds (weight not money) attached to him in the form of his suit and all the equipment he carries on his tool belt - i would love to see anyone swim comfortably with all of that crap on them!
I've spent a lot of time exploring Arkham City and doing side missions and have probably spent 2 hours on the campaign, so far i have not lost interest in this game in any shape or form. The campaign has some new characters thrown in and some appearances from those who were in Arkham Asylum.
Do yourself a favour and buy this game now!