Played the game for about an hour and half last night (GMG keys were distributed ~ 11PM EST and the game was unlocked immediately once downloaded. Initial thoughts are as follows.
1. Gameplay is very, very similar to FC3, which is a very good thing. The grapple is done very well and I think its going to add some really interesting gameplay elements. Pagan Min - like Vaas from FC3 - is a friggin awesome villain. Can't want to see more of him. From the map the game world is absolutely massive, and like FC3 most of the locations are filled with things to collect, see, or do.
2. The graphics are pretty nice, but IMO are not a huge step up from FC3. Noticeable texture improvements abound and the game is quite pretty, but it is definitely not the most beautiful game around. That said, the game surprisingly seems to run pretty bad on my rig (Win7x64, 16GB ram, 780TI, 3770k at 4.2Ghz). It is defintely not the buttery smoothness that is FC3, but maybe that is to be expected given the graphical "improvements" that are being implemented. Turning off msaa helps a lot, and doesn't seem to hurt image quality that much. Didn't have a chance to play with too many of the other graphics settings.
3. Mouse and KB controls are completely F'ed right now. Mouse acceleration is present even when it is configured to be OFF in the config menus. Editing the config files does not help. Hopefully they will patch it, as the game is virtually unplayable with mouse and keyboard. Using an Xbox controller works fine, though it comes with all the limitations of using a controller to drive a FPS.
4. Driving is a complete chore. Some dumbass thought it would be a good idea to map forward and reverse to up and down on the left stick, with instantaneous switching between the two at even the slightest movement either way. This results in extremely unpredictable driving mechanics, particularly on the ATV's. I have not investigated whether it is possible to remap the driving controls yet, but if not that is something that absolutely should be patched in. That being said there is an autodrive mode that helps quite a bit.
All in all I like the game because it is more of the same goodness that was FC3. But there are quite a few points of frustration that make the game "less" than what it could be.