Assassin's Creed
The sandbox approach in Assassin's Creed provides decent entertainment. The only question is: for how long That would depend on your preferences, really. If the recurring mission structure doesn't bother you, this could prove to be a fun ride all the way.
Looks and sounds awesome; new missions add a much needed variety to the gameplay; decent controls
Hardware requirements exceed those of Crysis; certainly not entertaining enough if you've already played the 360 and PS3 editions; AI quirks; recurring mission structure