Armed Assault is the most realistic military simulation to date, and fans of Operation Flashpoint shouldn't miss it!
As far as gameplay goes, I would also have to say that it is quite impressive. The missions seem realistic and you really get the feeling that you are participating in a real military operation. While some of the missions can be brutally hard, it is extremely rewarding when you finally find a way to complete them. While there are a few bugs with scripting that may cause necessary events not to trigger and hose up a mission from time to time, I haven't had too much trouble with this, and I am sure it will be address with a patch.
Multiplayer impressed me as well. while it's not quite what I was hoping for, I can see that it has a lot of potential. I joined a game or two, and I was teamed up with some good players that worked as a team and gave me the feeling that I was part of a well-oiled special ops team that was dropped in the middle of a combat zone in order to destroy some enemy armored assets. While the quality of the multiplayer experience can hinge on the experience of the players you are with, I find that it is VASTLY superrior to that of Operation Flashpoint.
Overall, I really enjoy playing Armed Assault. I have yet to finish the campaign, but I can't wait until I do. Even though there are some very difficult missions that require a lot of patience, thought, and skill, it's a very enjoyable simulation for people that want to get close to the battlefield without taking a huge chunk of lead to the face.