One of the biggest problems facing Open MP Betas like this is that everyone and their dog tries to host a server on their own machine because they have "a bitchin' cable modem" or whatever, not realizing that their upload is throttled to 128kbps or so - which can only support about 5-6 players of any recent multiplayer game.
The result is horrible horrible lag on a server you initially have an excellent ping to.
IGI2's ping detection eventually kicks everyone off the server because their ping goes through the roof as the host machine becomes incapable of sending out all of the necessary packets.
I'm not saying this was your problem, but I've had a fairly enjoyable experience with this MP Beta.
I did make sure to only join servers marked with a "D" in the server list, indicating "Dedicated".
I was running with everything at 1024x768 at the highest graphics settings, and getting very playable framerates.
System stats, for comparison:
AMD XP 1800+, GF4ti4400, 512MB DDR RAM, SBLive! Value