I bought it last weekend, so far loving it.
My own personal opinion is that it's a more realistic and more adult version of Just Cause. Play area is huge, and although I'm only 12% into the main game, have been doing a lot of the side missions.
I can understand why people didn't like it, it's nothing like the FarCry in terms of special powers or anything, this is more pure mercenary missions in Africa (taking out people, fuel depots, supply trucks, resucing hostages, etc), and while I'm sure in the real world AK47's, MP5's and other weapons aren't 100% reliable all the time, the irony of having your target in your sights only to find your weapon has jammed and you've to press X to unjam it, can at times be a little annoying.
But overall, I love it, gorgeous graphics (although did see a flying Zebra last night - some glitch somewhere!).
Not touched the MP side yet, so can't comment on it, but definately my type of game.