After a few partial runs, I finally finished my first Cataclysm heroic last week. It was a guild group, Throne of Tides, and we had great fun. But it took us 3 hours, due to a combination of being extra careful, using crowd control, and wiping several times. I've been to raids that went faster and took less effort.
Now I don't mind Cataclysm heroics being more challenging, but 3 hours of uninterrupted play session, with the need to remain focused and concentrated all the time, even on the trash, is not something I'm able to do every night, after a hard day at work.
I notice that this is a general trend in my guild. In spite of us being a "raiding guild", albeit not the most hardcore one, there is relatively little heroics activity right now in the guild. Many people spend significantly more time doing daily quests than running heroics. As a result progress is generally somewhat slow.
That might not be a bad thing, I'd rather have slow progress now than being already burned-out from heroics in a month. After all, Cataclysm will have to last us for two years, and the amount of new heroics that will still be added with content patches will be limited. I presume progress will be auto-accelerating: We slowly get better gear and experience with all the heroic encounters, which leads us to running heroics faster, which leads to us getting gear faster. So even if progress is slow now, the outlook isn't all that bad.