There are several ways to play RPGss online, and all have their advantages and drawbacks compared to offline play. I've had experience with most of them, including the old-style Pern PBeMs, some Star Trek experience, participation in several prominent LJ RPGs, some forum play, and a D&D campaign over AIM chat.

In my experience it's a little less ideal and takes longer than offline play even at best, but it has its advantages too, especially if you get away from the dice and do more drawn out storytelling.

Scheduled chats are closer to the offline experience, while the PBeM side tends to have long conversations between characters stretching for weeks and several things going on at once.