I think, as a long-term lurker, I may emerge at this point to be the first person to receive the physical Dark Ages in the post and play it? I haven't been a beta-tester, so this has been my first exposure to the set.
My impressions after one game, which may not be worth much, are:
(a) The theme is side-effects going off almost at random, which can feel like walking through a fireworks factory which has itself caught fire. This set was absolutely born to be played on Isotropic, with a really reliable engine resolving everything correctly, and a minimum of book-keeping. In my one game, I persistently had 7 or more Actions, frequently losing count since they arrived via Procession and thus were not tallyable using a binary tree of cards on the table; and I typically held an entire deck of 40 or so cards in my hand. It's not satisfying to be saying "well, I've lost count, but I've got lots of buys and actions now" so often.
(b) Procession on Fortress is so powerful that we just couldn't believe it, and read the rules and the booklet many times. These are cheap cards: 4 each. I bought them on early turns, and essentially won the game as a result, 8 Provinces to zero. The effect is this: Procession plays Fortress twice, giving you +2 Cards, +4 Actions; Procession then trashes Fortress, but Fortress goes back in your hand when trashed, so you don't lose it (and can play it again this very turn); and you gain a free cost-5 action card into the bargain. Make that card a Band of Misfits, which can pretend to be a Procession, and repeat. And repeat. Remember, you only need one Fortress in hand, and then any Processions or Bands of Misfits can be played over and over to repeat this in the same turn.
Very soon you can reliably have colossal quantities of everything. Procession is enormously stronger than (say) Golem, yet costs the same as Throne Room. We read the rules for Procession over and over trying to find a reading in which it came out weaker, but in fact, these rules mostly just emphasised its strength, giving a maximal interpretation to what happens if it combines with Duration cards.
If anyone thinks Procession plus Fortress is an unlikely combination, I point out that it's in the recommended first play selection.
(c) There are, all the same, many beautiful pieces of rule design and artwork here. The Ruins are a pleasing reminder of the base set cards as if they have decayed for centuries (ironically, since this is meant to be the Dark Ages and so to precede the base set chronologically, I suppose.) The novelty of the three Estate substitutes in the opening hands is ingenious and fun, since it means that the traditional two bits of information on the opening hand now opens up to many more possibilities. All the same, these delicately weak cards are overpowered by the unduly strong ones, or so it seems at this point. (Band of Misfits, which I mention above, can pretend to be any 4-point or cheaper Action card of your choice. And several cards give you rather more draws from the deck than you might expect for their price.)
So I'm afraid I have to report that our first game of Dark Ages was unsatisfying. After about turn five, we simply didn't have that good a time playing it, even though the Knights were fun, and simply discovering new cards after so long playing the old ones on Isotropic was quite a thrill. Can't really have an opinion after only one game, of course, but my initial reaction is that pure Dark Ages may be an overpowering brew, and that the thing to do may be to mix it half and half with some other set.
(I do like the artwork on the new base cards set, I must say.)