Multiplayer is superb!
There are tons of game types such as capture the flag, sabotage, deathmatch, etc. The best part is, you aren't subdued to only the maps they give you. You as a player can also create levels. You can customize the team's characters, what game type it is, where the objective is located, the weapons you can use, the obstacles, the weather, and much more!
Of course there are clans, rankings, etc. which keep you playing and reaching for another goal each day. The only real flaw about Combat Ops is that there isn't that much people playing. The most people playing at once each day is probably 100 people. But don't let this stop you because I manage to always find a game unless it's the time of day where practically no one is playing.
If you like online multiplayer shooters, you own a PSP and you have internet access, this is a must buy! And even if you don't have any of those traits, you should still buy Syphon Filter: Combat Ops.