Arg, most of the way through the post and my browser crashes on me. Let's try this again.
I've played exclusively on IRC, so I can answer this question! Combat goes as quick as people are into the battle, as the speed is dependent on each player paying attention to the action and responding when it's their turn. Descriptions in combat tend to be short, as the time to type it out is time when the combat is going slower. We tended to use homemade ASCII maps, since we could never find a good satisfactory and affordable map program. The biggest awkwardness was the conversations, since people type at the same time without knowing what or if anybody else is talking. This can result in some awkward conversations and redundancies, but it's actually not too hard to work with. As far as descriptions go, it's good to have the DM have pretyped ones to keep the action going. Most players give good, but not overly time-consuming descriptions with emotes. Not sure how much I can say roleplaying would be better or more fun, to be honest.
As far as definitely good things goes, for one, I like having a good dicebot around; you just plug in what you want and you get the results right there. No counting up 10 d6 dice, no "bad dice", and no falling off the table. It's a lot easier for the DM to hide stuff, and if a DM wants to send one of the players a private message, the other players don't necessarily know that it happened. OOC chat can also happen during the game in another channel and not interfere with campaign as it goes on (as long as the players don't get too involved in OOC chat anyway). Then there's logging, which lets you keep track of everything that happened down in precisely what order, so if anyone forgets they can just go back through and read it through. Char sheets can be kept track of in a simple, easily updatable file format (I always just used txt files). And of course, it's possible to play when one of your player friends lives in Florida, one in Kansas, one in Washington state, one in Texas, one in Pennsylvannia, and one in England (these were the people I most regularly played with).