First impressions of the new cards, having played a few games with them:
Harbinger
It suffers a bit from Settlers Syndrome: If you're engine-building, it's a do-nothing card, because a huge amount of time you have no discard pile. At least Settlers works with sifting because it increases your handsize once you've discarded a Copper and pulled it back. Sifting and Harbinger is almost completely useless.
It might end up being a decent card in Slogs or Rushes to re-use key cards, on the other hand. I used it in a Rebuild game to topdeck Rebuilds or sometimes Silvers if I needed to hit $5 for another Duchy.
Merchant
Nothing to report yet. It hasn't been a key card in any kingdom we've played so far, but I can't imagine it won't be. We just had another Peddler game that came down to Peddler mania, even with only a trash-one (Then again, two Foragers and Bonfire will turn your entire deck into a Peddler party pretty darned quick.) Merchant will have its day.
Vassal
My new favorite terminal Silver. Oh, wait. It's not usually terminal. Seriously, it does an amazing impression of Conspirator. All you have to do is get it a little help, and there are a LOT of cards that help it, including the new kids: Sentry and Secret Passage! It's so great playing a terminal Silver, turning up a Cantrip and then continuing on your merry way.
Also, it's a very happy King's Court target, even without deck manipulation. But then again, what isn't a happy KC target?
Poacher
Very interesting card. I watched several players buy at least two, and nobody wanted to be the first to pile something out. We ended up with several one-card piles. I'm going to watch for TfB the next time it appears, because I feel like I might start the Poacher rush, then TfB mine and buy the last one just to inflict Oases on opponents.
Bandit
Gain a gold is interesting. In 4P, everyone's so paranoid about losing treasure that they all play treasureless engines if possible. Definitely a huge upgrade over Thief, but you'll have to ask yourself how much Gold you want.
Sentry
I had high hopes for this card, but I'm having second thoughts. At first, I though it would be a better trasher than Junk Dealer, and it emphatically is NOT. It has only two cards to choose from, and JD has five. Getting it to match up with junk is surprisingly difficult, especially because it's a $5 and you've bought four keepers by the time you play it even once.
That said, I do have to say that the discard/inspection/topdecking capability is perhaps the most important thing about this card. It enables Vassal, Mystic, Wishing Well, Herald, Tunnel, etc., it skips the green, and it's non-terminal. It's actually a pretty great utility card. It's just not an elite trasher.
Artisan
Came up in a Rebuild game. Believe it or not, it wasn't relevant. Maybe it's because I just didn't try hard enough to hit $6, but after buying some Rebuilds you really have to hammer the Duchy pile. I really wanted to Artisan a bunch of Duchies, too, but I won without an Artisan at all. That said, it's a great $5-gainer, and if you don't have extra actions, putting the gained card on top is especially nice to avoid the collision. That seems to be predominant play for this card so far: Gain/topdeck the gained card.
Lurker
Nobody in my playgroup is sold on this so far, but the interesting thing is that as soon as one player buys one, others jump in and buy one to try to snipe the trashed actions. No one has yet built an engine and subsequently bought two at once.
Diplomat
Hasn't been great so far. Some tried handsize-decreasing to enable it, but it's been difficult to get it to match up with handsize-decreasers because you don't want too many of those. It could theoretically work with optional handsize-decreasers like Mill.
Mill
Money for green is relevant. However, it's not good if there is other sifting, because you're already discarding your green. For example, I tried one Mill in an engine that also had Embassy. Nope.
Secret Passage
Possibly my favorite new card. I love that it enables so many other cards that care about the specific location of a card in your deck, especially ones that care about the SECOND card in your deck. I can't tell you how many times I've looked at a top-decking card and thought about how it might work with Herald or Wishing Well...nope.
I also really like sifters that don't decrease your hand size. Forum and Archive are great. Secret Passage is right up there.
It's also relevant to smooth out draws; sometimes you bottom-deck a green card or several, knowing that you're going to get your draw components quicker and re-draw the green anyway, but after you have your engine pieces in hand, rather than tripping on them along the way. Or you can put the green second from the top and turn your Vagrant into the Lab it so desperately wants to be but usually isn't.
You also have to love that it enables new cards like Sentry and Vassal.
Courtier
A bit underwhelming. Flexibility is good, though, and it's been relevant even in games without multiple-identity cards. Gold gaining can be quite good. Sometimes you really need a +buy that badly, and sometimes you really need $3 that badly. It's almost always worth buying one. ONE. Don't get carried away.
I have to say it can be hard to evaluate cards depending on the type of game you're playing. Courtier and Bandit, for example, get quite a bit better in a Palace game. You can lose out on points by not having enough Silvers!
Patrol
I would buy Smithy at $5. +3 cards is just that good. Sifting Green while you're at it is great. This card slaps Scout's stupid face right off. No surprise there.