I've played literally hundreds of RPGA games online, using AOL Instant Messenger. (The group I've played with has thrown around the idea of using OpenRPG, but, by all accounts, it's got a steep learning curve, and some of our regular group (myself included) aren't very tech-savvy, or are playing behind corporate firewalls, which can be challenging.)
It's not a perfect replacement for face-to-face play, by any means, but it's a reasonable alternative, IMO. The folks I've seen who've tried it and don't like it tend to just not like spending an evening sitting at the computer anyway.
Yes, typing is slower than talking. Most RPGA modules are written to fit in a 4-hour convention slot; we find that most of them take 6-7 hours to play online (and that's with the DM having a "cracked" version of the module PDF, allowing him to cut-and-paste boxed text). Typically, we'll plan to play a module over the course of two weeknight evenings, playing for 3 or so hours each night.
We generally don't use a battle map, though there are a few occasions when a combat's too complicated to go without. In those cases, we usually use a numbered / lettered grid. Works OK, though that's a place where a dedicated online gaming program, like OpenRPG or Fantasy Grounds really has an advantage over AIM. It's the combat-intensive modules that seem to play the slowest.
We have quite a lot of role-playing, but then, we usually play with a core group of players that all know each other, and have fun RPing together.
One thing that we do find problematic is when you get into a "question and answer" session with an NPC. Players can type in questions rapid-fire, and it buries the DM. Some DMs in our group have to rein the players in a bit on those.
AIM has a diebot, but it's notoriously fickle, and some of the programmers in our group claim that it's not really random.