I have used several different ones and it seems none of them are consistent. I will say to try to avoid the ones with the advertisements in a window you can never close; Netzero for instance I have used and the ads are still visible while playing. Even at a high resolution and moved somewhere out of the way it is still distracting and of course the opposite of immersive.
I have been using Freewwweb for some time now and it seems to go in streaks, sometimes it is a great connection, other times I cannot even browse web sites or upload new web pages because the connect is so irregular. There are no ads and no software to download, you just set it up in dial-up networking.
FreeDSL is one that I have just started using. I don't know if they have actually activated the DSL service in any area yet, Los Angeles was supposed to start this spring but I haven't heard anything yet. They provide free dial-up access with no ads until they hook you up with the DSL. It seems to take a long time to logon but it is pretty smooth once connected.
I have a winmodem and I don't know the quality of the phone line to the computer (it's an older house), so some or all of my problems may be more related to that than the ISPs. As far as playing the game, during the beta it seemed I would disconnect more often than others mentioned. It works okay H2H, warpy every so often, but usually a relog takes care of it. I play WB H2H with my dad in Portland, OR once a week or so and it works fine for that once I'm connected.
None of them seem equal in quality of connection to a pay ISP (although I haven't had one for a couple years), but it is cost effective.
LuckyDay