Having lost my entire Saturday to this, I'm really enjoying it, BUT...
Graphics - awful. I picked up the 360 version rather than PS3, as the 360 sometimes has a very slight edge over the PS3's graphics... calm down fanboys of both, its a personal judgement from someone with both consoles so get over yourselves. You do learn to see past them for the game beneath, but as the bulk of my time is spent patiently waiting for the perfect time to take someone down, you can't help but sit there thinking "god, those textures look terrible".
'Sandbox' - it really isn't, sure there's a bunch of ways to get about the level, but when you explore every inch, you realize that you're just in a cunningly constructed tunnel (albeit a large one), which is designed to route you to the next loading door, taking you onto the next cunningly constructed tunnel. This is how I feel the city areas to be - I was hoping for literally an entire city, where I could actually go about it any way I wanted, but it's not the case.
Overall, it's feeling a bit too repetitive - I've done two assassinations and I'm on a kidnap one now, but I can't help feeling that I've been here and done this before.
Don't get me wrong, I really enjoy the game, and would happily recommend it to anyone, it's just.... well, it's no Thief.