Pros: Great levels of realism/authenticity, no silly arcade features, not casual, teamwork emphasis
Cons: Single player campaign hardly exists, but that's a given
The realism and authenticity bring intensity levels above and beyond all other PvP shooters. It's not an all-out simulator like ArmA, and you'll find that game modes aren't too much different than what you see in games like Battlefield. RO2 features large, carefully planned out levels for balanced strategic gameplay (something Battlefield lacks), and features ground vehicles with excellent realism and control. The driver no longer drives the vehicles and controls all of the weapons, so even the task of driving and operating a vehicle becomes a test of teamwork.
On realism mode, RO2 also restricts the amount of players you can have in every class, for a realistic and balanced team. Remember how you'd find like 20 snipers on your team in Battlefield? That won't be a problem here.
Shooting mechanics are some of the best out there. The ballistic system is wonderful, it does a good job feeling authentic unlike Battlefield (which tries and fails miserably). This combined with the aiming deadzone, excellent sound effects, range adjustment, usable bi-pods, and crisp emulation of recoil, make shooting people more satisfying than just about any other shooter. Unlike ArmA 2, mouse movement and support is perfect with no forced mouse smoothing.
If you like PvP shooters, definitely pick up RO2. The playerbase is much smaller than mainstream games, but is still large enough to find fully occupied servers easily. The game supports 64-players, and does so better than BF3 (with much better hit detection as well). And unlike many modern, mainstream PvP shooters, RO2 features a complete in-game server browser. It really hits the mark, and has very few flaws as a PvP shooter.
Full review here