Here's what happened to me:
Started the demo from Steam, watched lengthy promo/tutorial.
It could have been summarised thus:
E = Stealth
Q = Armour
The suit energy level depletes with use
Watch out for power-ups & bonuses.
Went to try demo, as far as I can recall I've never had a gamespy account so I thought I'd create a new account.
15 minutes later the 'Creating Account' window is still up, I couldn't exit so started Task manager and shut crysis demo down. This gave me a BSOD.
Try again, this time my router froze up, red led instead of a a green one. Disconnect DC plug and power up/reset router again.
Tried again, managed to skip the tutorial and attempted to create an account again but it tells me my e-mail address is already in use so I request password retrieval as I have no record anywhere of an account with either Gamespy or Mycrisis.com
'Server busy'
Not really impressed so far.
And I didn't realise it was a multiplayer demo otherwise I wouldn't have bothered, I'm only really interested in the single player game at this stage.
I'll wait awhile and see what everybody thinks of it before purchasing.