I'm at Origins and got three Guilds games in at the Rio Grande booth. Here are my initial strategy impressions.
Coins: Having one coin producer in your deck is a good boon. Not shockingly powerful, but you'll notice that you just don't tend to draw the wrong cards. Having many coin producers is much less important and there are almost certainly better things to buy.
Butcher: Possibly the best card in the set. Don't fall into the mental trap of thinking that the 2 coins you get should be put right back into the remodel. Ideally you keep or spend those coins while performing an equal cost remodel, like Curse-Copper, Estate-Candlestick Maker, or last shuffle $5 action-Duchy.
Plaza: People wonder how this compares to other $4 villages. Well, the first one you buy is the best $4 village, and after that it is the worst. Discarding a treasure is an expensive way into get a coin! It will work nicely with draw-up-to if those cards happen to be present but won't be a dominant strategy without something else.
Baker: You know how novices, after the Village Idiot phase, try buying a bunch of Markets, but that deck is just too slow? Baker lets you fall into the same trap. Like Plaza, the first one is good, after that they drop off significantly and you need to find something else to do with $5, like toss in a coin and get Gold.
Candlestick Maker: This is a good $2. Not at the Hamlet level, but usually better than say Pawn I think, even if you have several. The coin is helpful, the buy can be too. It's only one coin, though, so don't go crazy.
Merchant Guild: I'm not really an engine player and it's difficult for me to imagine the sort of board on which this is viable. By itself, it's expensive and clunky. I can't see picking it up unless you happen to need the buy anyway--maybe a Highway engine?
Masterpiece: Rarely will you buy more than one in a game. Turns those early freak $7 or $8 hands into a Silver bonanza rather than a disappointing Gold; lets you green a bit earlier. Good protection from Saboteur/Knights/other trashing attacks. Barring Feodum, this is just an opportunity that you exploit, not a cornerstone of your strategy.
Doctor: Lookout is the key card to compare Doctor to, and thanks to being terminal, I think the comparison is usually unfavorable. I saw it overbought several times and each time was happy that my opponent had done so rather than just buying an expensive card.
Herald: Although I bought several, I don't think I exploited them particularly well. I think Herald's home is an engine that runs lean on villages. Otherwise it's an expensive cantrip. The ability to topdeck village + draw, though, does synergize well with the sort of decks Herald wants to be in.
Taxman: I think this is the other contender for best card of the set. It really is a $4 Mine and compares favorably to Moneylender thanks to the attack. I think it's always a decent opening, but really shines if there is a good $2 so that CCCET is not wasted. Upgrading Silver to Gold is risky unless there is a good chance your opponent has a Silver to nail.
Advisor: I think this is a bad card and in particular a terrible opening. So many times you will wish you had bought Silver. Think of a Caravan that always skips your best cards. I can only see this working out in a deck with good sifting (Warehouse?) that is desperate for hand size increase.
Soothsayer: This is an good card. You know how Marauder is good? Well, Soothsayer gives worse junk and gives you real gold. The tiny hitch is the Council Room effect, of course, and don't bother trying to combo it with Militia. If the opponent has good trashing and a use for big hands, Soothsayer might be skippable.
Journeyman: Although this card was available, I rarely saw it purchased. I think it's worse than Catacombs. You'll buy it if you need draw bad enough to pay $5 to draw three.
Stonecutter: Similar to Masterpiece, this is a big deal in that it let's you turn $7 into two good cards. And a Stonecutter, though, so what good is that? Late game, cracking Gold into two Duchies is dynamite. It doesn't really help you lose the real dross, though.
Edit: corrected Market Square -> Merchant Guild. Oops.