Cataclysm has only been out for a little over a month. By now I've run all the new heroics at least once, but still less than five times. With that said, I want to put down what I guess could still be considered early impressions of them.
My overall verdict is: I like them. Their length feels good, and unless you're playing badly and wiping a lot, they don't take much longer than WOTLK heroics did early in the expansion.
I like the trash. Even though a lot of it has been trivialised way too quickly, there are still some interesting pulls in there. My favourite are probably the gilblins in Throne of the Tides. I still remember the first time we did that instance on heroic and our pugged tank tricked my boyfriend's warlock into pulling with a fear. My dear man was rather annoyed by getting one-shot by all the spears aimed at him but I have to admit that I was amused more than anything. I like the way these pulls encourage thinking out of the box and pulling in unusual ways: with a mind control from the healer, a mage ice block or by sending a pet in.
More importantly, there are generally what I consider "healthy" amounts of trash as well. A couple of packs between bosses to give you some room to refocus but not enough to turn it into a tedious slog. There are some exceptions of course: For example Anraphet's room in Halls of Origination bores me a bit to be honest, as does the amount of trash just before Ozruk in the Stonecore (though the latter is at least dangerous and you've got be careful with pulling). On the other hand the final four bosses of Halls of Origination and the last three of heroic Deadmines feel a bit too "piled up" too me, with little to nothing in-between to let you relax a bit.
The bosses themselves are without a doubt the best thing about Cataclysm heroics, because Blizzard clearly tried very hard to make all of them interesting in some way. As nostalgic as I am for Burning Crusade sometimes, I'm under no delusion that all the level seventy heroic bosses were interesting. There was a lot of tank and spank even back then. In Cataclysm heroics however I'd struggle to name any bosses that feel completely boring right now, maybe with the exception of Grand Vizier Ertan and Asaad in Vortex Pinnacle, and they are both about to receive buffs in the next patch.
Personally I also find the difficulty of most bosses to be quite well-tuned, and I think that a lot of the panic about heroics being too hard is simply the result of ignorance and lack of co-ordination, or in other words, we're still so early in the expansion that you're likely to have at least one group member who has no clue what to do and might blow everyone up. I'm pretty sure that if the dungeon finder had been around at the start of WOTLK, people would have whined about those heroics being too hard as well, and we all know that that didn't last long at all. So I believe that this is something that will sort itself out over time. There is only one boss that I'd consider somewhat overtuned right now, and that's Ozruk in heroic Stonecore. I've only tanked him on normal myself, but even in there and with a run-speed enchant on my boots I found that a half-second of delay was already enough to make it impossible to avoid his Shatter properly, which will generally one-shot you on heroic in current gear levels. I think this could be fixed quite easily though by simply increasing the cast time a little to give the tank and melee a bit more leeway with running out.
If I had to say anything negative about Cataclysm heroics, it would probably be that none of them have really impressed me visually. They are far from ugly, but unlike in BC and WOTLK I never had any "wow, this looks cool" moments in there as it stands. Even Vortex Pinnacle, which some have praised for its looks, just left me with the impression of a place that was very white and very empty.
With all that said, I would like to note that I'm not running a heroic every day, which some people have cited as a problem with the current system, because apparently anything that you wouldn't want to do every single day is bad. Maybe I'm an oddity, but I didn't do the daily heroic every day back in WOTLK either. Occasionally I'd go on binges and run a daily on all of my alts in one evening, but more often than not I simply couldn't be bothered because it was simply kind of boring, especially towards the end of the expansion. Nowadays lack of time keeps me away instead of a lack of interest, but at least when I do get to join a run it's somewhat engaging.
Right now the gear that drops from the heroic bosses themselves and the things that you can buy with justice points are all you really need to gear up for raiding, and neither of those are dependent on running heroics on a schedule. Valor point epics are only a nice bonus anyway, so I see no reason to fret about them.