Its a great game, if you can pick it up for 30-40 bucks then do it. The sandbox is a blast to explore and is HUGE! Like seriously huge. Its fucking beautiful, some of my favorite graphics in a game, so colorful. The gameplay is fun as hell at the start, but it does get a bit repetitive, and the size of the world means you will never really use cars or bikes, mostly helicopters and jets.
It has a huge collectible system. Like each little area be it a district of a city or an enemy base or a small town has a number of things to either collect or destroy. Now if you are looking for bang for your buck, you could spend weeks finding all of these, 100% in this game is practically unobtainable. The missions while off the wall and fun are a bit repetitive, especially the strongholds, and all the cut scenes suck pretty much. But it is an awesome sandbox, one of the best. The grapple hook is the best thing ever as well.
Overall I would say buy it, but depending on your attention span to playing a game like this and exploring everything you could easily see yourself putting this one down after 15 hours and only 15% completion.
Buy it, buy it for 40 or under and have fun.