Can't say I'm a huge fan. Typically, I only enjoy them if they have driving and lots of crazy over the top stuff to do like Saint's Row or the PS2 GTA games. Rockstar's open world games are boring this gen, IMO.
Reading: The Great War for Civilisation: The Conquest of the Middle East - R. Fisk http://i.imgur.com/AUbFA.jpg http://i.imgur.com/nYklT.jpg
I am usually impressed with the insanely huge and detailed map, and enjoy goofing around doing random things. Like driving in GTA while listening to the radio. That doesn't last long in my case, though. Eventually I want some focus and they don't do a great job at that. The missions are always meant to show off the giant world (go to the other side of town and destroy this, escort/chase that person, protect someone/something, race that guy), which gets old fast for me.
If only the open world was complemented by something like the Zelda dungeons, with their own level design and a bit more focused. Then I'd have something to look forward to in these games.
OP, have you played Just Cause 2 yet? For me that's the best sandbox game this gen fun factor wise. You don't even need any weapons to have fun, your grappling hook is all you need. Even something as simple as falling down a mountain as a ragdoll can entertain me for hours. Not to mention it has the biggest map of a sandbox game ever.
saints row 2, skyrim, and now borderlands 2 are the games i've spent the most time playing. only other genre that comes close is fighting because of UMvC3 and TTT2. rage was also good. but not really sandbow nor open world... it was like playing doom outside. them open world games, especially with a good loot system really can become a time consuming hobby. i pick up the controller at 10:30am, start playing borderlands 2, level up twice, look at the clock and its 5:00pm and i'm starving and need a cig. its crazy how intensely i get into open world games... theres no real pressure to do things on time, but you just keep going and going.
a coward dies a thousand deaths... a soldier dies but once. what do you think i am, a euthanasia service? -Iori, King of Fighters EX2