I hadn't heard of gmail before today.
I found out in the early hours when I logged into one account it was (omgwow
!) 100mb... and one by one the other 6mb accounts later followed suit. They (Yahoo) do offer an upgrade for something like $15 a year to 1gb of space but after using 6mb for so long I probably won't even use the 100mb for a while. Strange, really.
I'll probably even delete one of my multiple old yahoo accounts that only get spam now.
From what I hear, the general consensus is that it was a reaction to Google Mail (gmail) that led to them increasing the inbox size. Co-signing on the fact I'd deleted stuff the day before because I just "didn't have enough room".
It hasn't been slow etc for me, I think they updated accounts to 100mb in tranches - e.g. my old rocketmail (bought out by Yahoo) was the first to update. I did have problems at the start though, but I think they're on top of things now like others said, above. *crosses fingers*