BioShock 2 was a lot of fun. Rapture is even more artistically beautiful and intriguing than in the original. The world is full of details making for a constantly absorbing experience. The enemies are fun to fight and give you plenty of incentive and opportunity to experiment with different weapons and plasmids. Upgrading your character makes you feel powerful and the enemies make various satisfying noises as you dispatch them.
The spear gun was great. Pinning enemies to walls is a lot of fun. With upgrades, it works like a sniper rifle. However, I found that the game suffers from poor collision detection at times which results in your long shots simply disappearing along the way. You quickly get the hang of dual wielding plasmids and weapons. Certain combinations are obvious and make the game quite easy. You may find yourself varying attacks just to keep it interesting instead of just trying to stay alive.
I really liked the new research method which encourages you to try out multiple plasmids, weapons, and attacks to get the most research points. In addition, I found that upgrading was easier (i.e. there was more ADAM available and items were cheaper) making it possible to create a huge combination of tonics and plasmids.
The additional moral decisions you make along the way are a nice touch. However, they pale in comparison to the depth found in games like Mass Effect and Fable. I occasionally found myself doing things that felt morally ambiguous and wondering if it would affect the outcome. They didn't.