I started bnet about 3-4 weeks ago. It's been great, possibly the best D2 experience I've had. Don't get me wrong, I still enjoy mods and modding, but the interaction with other people really adds to the game (even with the idiots).PvP also adds tremendously to the game. There is a certain "rush" that comes over you when you are in a duel, especially in hardcore. The only problems are the obnoxious PKers, which haven't really affected me yet.
Also, certain areas in the game can be incredibly tough for certain classes, especially in multiplayer. The answer to those areas is partying, which is not available in single player.
Lag, for some reason, has not a problem with me, even on a 56K modem. I used to get the occasional lag spike, or the even rarer disconnect, but for the most part, it felt just like a single player game. Now that USEast has been doubled, I haven't had a single problem in the 3 days since the upgrade.
Overall, I'm really glad I tried out bnet, although it seems like I don't get as many phone calls as I used to.
I really need to get a second line if I continue to play multi.