I gave MAG a go last Oct/Nov, not sure if they've updated it since then, but it is a bit naff isn't it? Yes, it is impressive they've managed to cram so many players into a game but it hasn't made the game better or more fun than your typical FPS. You don't actually get to see that amount of people on the map, you just battle away in your little part of it with a fraction of the players.
The majority of games I played usually involved the two teams being locked in a stalemate. With one side defending an objective and the other attempting to take it. But with there being so many players in such a small/open space neither side really gained any advantage and were left to slug it out for what felt like an hour, WW1 team deathmatch stylee, until the tickers ran out. It was a bit like being stuck in an old COD game against the AI. If you didn't progress on a single player level the game would just keep throwing soldiers at you until you moved past a invisible checkpoint. Except there was no checkpoint... just endless waves of targets.
Occasionally I'd also get a game where one side, for whatever reason, would be able to totally overwhelm the other in no time at all. In one match I just sat in the back of a truck as we drove to all the objectives and won the game in less than 3 minutes. Which was a bit weird with both sides being full of players.