I experienced my first co-op gameplay with MERCS 2 on Xmas eve. The other player didn't have a mic, chatted back and forth with me via the unwieldly XMB via USB keyboard. Co-op is only good if both players know what their objective is. Unfortunately, the XMB's built-in chat is a terrible way to discuss mission tactics. Oh and there really were no actual tactics. I seem to attract random players who aren't sure in which direction to shoot >_<.
No more random co-op for me.
Killzone 2's offline skirmish mode is looking really enticing about now.