I promised myself that I wasn't going to get into this, but oh well.
Operation Flashpoint and Ghost Recon are both decent infantry sim games, but GR seems more thought out and streamlined than OFP. The main problem, IMO, with Flashpoint is that it's (brace yourselve) too non-linear. There are so many ways to attempt to beat a given mission that its often difficult to play the game the way that it was designed to be played (to some degree). Other problems with the game include the craptacular squad/platoon controls, the strange UI, the robotic voice acting, and running around for an hour trying to find your objective only to be shot by some randomly placed enemy and having to restart the mission all over again because your only allowed one save.
The multiplayer for both games suck. Since both aren't client/server, there is usually a waiting period until the next game begins. Due to the massive uncoordination and gigantic maps, the usual wait for an OFP game is around half an hour. GR's is anywhere from 5-20 minutes.
I can go on and on about this, but it's late and I'm tired.