Well so far it has been great, although I haven't even left the starting islands yet (though I did just finish liberating them entirely, so I'll probably be heading off soon). Since this often feels like an extension of JC2 which I played forever, it is real easy to just relax and take my time in it. There are a few minor annoyances, like all the upgrades being locked behind various challenges, a couple types of which are kinda boring/tedious or depend way too heavily on luck. But some of the other challenges are super fun and I would probably clear them even if there weren't upgrades gated behind them. The wingsuit challenges are a lot of fun, the races and destruction frenzies are mixed, the bomb runs are kinda bad.
Like the first destruction frenzy I came across was a sniper rifle one, it said to blow up as much stuff in the base as possible within 90 seconds, and the 90 second timer doesn't start until you blow something up. You are given your choice of your currently unlocked sniper rifles with infinite ammo to do the challenge. So I load it up, and start shooting at one of the big radar towers in the base. After about 30 seconds and like 5 clips of ammo, it finally breaks. And I'm like "ok, this is impossible, there has got to be a trick". So the second time around I decided to look around the area and see what options were available and I made a few important discoveries: First, grapple tethers and one full supply of grenades (3-6 depending on how many upgrades you have) are always available to you and work for the challenge (but sadly, your always infinite supply of C4 is disabled). Second, that military helicopter parked on the pad over there, yeah, the one with the infinite rockets and miniguns, well you can totally use that to blow up all the stuff and get a perfect score without ever firing a shot from the proper specified weapon.
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In a lot of the hardest destruction frenzy challenges, you can usually find a vehicle nearby that makes it comically easy instead. So don't hesitate to look around for a helicopter, gunboat, or tank somewhere in the base before you start the timer.
Also stuff blows up, a lot. Like a ton of stuff in the game is practically made out of nitroglycerin and lit matches. Watching a bunch of stuff blow up in a spectacular chain reaction is mesmerizing and dramatically improved from JC2. In JC2 all the explosions were basically "canned" explosions, every object type always exploded the same way, in JC3 however almost everything blows up differently every time you blow up one of them, so no two explosions are quite the same.
The online tracking thing is kinda dumb though, because when you log in and load the game, you are greeted by like 5 minutes of scrolling text in one corner of the screen of people who beat your score in blah blah challenge. And occasionally you do a challenge and get a "you are now rank blah in this challenge". It is kinda dumb, and according to people online it also consumes about 10 FPS worth of performance, so the game actually runs faster if you go offline and firewall it to prevent it from doing it. I can only guess it is from the game recording your telemetry data so people can replay your ghost and see how you did it. It runs perfectly fine on my machine, not a hitch or crash yet with some pretty long play sessions, but some people with weaker computers and a lot of people with AMD video cards are having major issues or crashes. So the main sticking point there is if you aren't running a fairly high end Nvidia card (GTX 780 or better) on a stable Intel CPU with at least 8 GB of RAM, you will probably have issues. My i7-3770k, 16 GB of RAM, and GTX 980 work perfectly, I suspect in a couple years this game will be running pretty much perfectly even on mid range hardware.