Yeah, I really like JC and JC2 as well. Loads of freedom. For me, it sorta loses out with the bad voice acting and dialogue, but when free roaming, it's tons of fun.
Oh and Gamgee, if you're interested, Red Faction and Merceneraries are also series' that are similar, in case you didn't already know. Merceneraries is pretty glitchy, but good for a weekend where you've got nothing else to do. The Crackdown games and Prototype games are similar, but branching towards the extreme, with superhuman powers and whatnot. Still fun though. Sandbox games are probably my favourite.