Bottle Imp:
Druid and Research, as well as a few other submissions, demonstrate a desire1 for an "alternative path to Potion cards." That's a reasonable aspiration. Potion cards are fun! Potions themselves are not.
But both of those cards illustrate the difficulties with the concept of "replacing" Potion. Druid, by both trashing your Potion and then seamlessly replacing it, takes all the "cost" out of Potion-cost cards. It's too obvious a solution to the Potion problem, and ends up oversimplifying gameplay, rather than branching it out. Research, on the other hand, is a redundancy -- a card you need a Potion to buy that then fills your deck with more Researches to get more Potion-cost cards. You'll rarely want Research if you've already bought a Potion.
Bottle Imp is, I think, the closest the contest came to a true Potion-alternative. I think it works because it doesn't aim to replace Potion. You need two action cards in play to even buy Imp2. So it will be even later that you finally get Potion cards into your deck than if you'd gone the straightforward route. Also, because each Imp has to be sacrificed to get a Potion card out of it, you'll never get as many Potion cards out of Imp as you would with Potion. So, while it's nice to get Potion cards without having to clog your deck with Potion, the drawbacks of Bottle Imp are appropriately severe. It's a true alternate route, and provides actual branching gameplay.
I don't think Imp is often a viable route, though. Potion cards are designed to be massable, and Bottle Imp won't usually let you mass them. So when you're going after mass Scrying Pools, Alchemists, or Apothecaries, Bottle Imp is probably worse than Potion.3 Familiar, on the other hand, you don't want to mass. But you want them ASAP, sooner than your opponent, and Bottle Imp is going to lose the footrace to Potion.
Bottle Imp is a nice card for picking up Golems, since you don't really want a Golem until you have a lot of actions in your deck already, and you usually don't anticipate getting more than a few of them. You might prefer Imps when Golem is the only other Potion cost card on the board. But, Golem also tends to replace the need for Alchemist and Scrying Pool, so, even with those cards present, skipping Potion might be preferable. Bottle Imp can also work nicely with Philosopher's Stone, a card that is more valuable later, and that you won't want more than a few of. And, in those cases where a Familiar is better later4 Imp is a nice Potion alt.
But, there are few cases when Bottle Imp beats Potion. Usually, Potion will be the best way to buy Potion cards. Luckily, Bottle Imp has other uses!
Someone commented that Imp's ability to gain Duchies is a bad reward for the hoops you have to go through to get one. But on engine boards, gaining Imp will be trivial, because you will almost always have two Action cards in play. In those instances, the ability to trash into a Duchy can be huge for the endgame. If you trash Imp on your last shuffle, then it was essentially a Market that was worth 3VP, which you probably bought for 5. That's a great deal.
For that reason, it's probably good that you can't trash Imp into Duchy if you already have two Action cards in play. That's the thing I really like about the card. The "two action cards in play" clause intersects with the card's value at two critical points. When the card costs 5, instead of 5P, it can no longer be trashed into Potion cards OR into 5s. Now it can only grab 4s. That might be nice for grabbing mass Silk Roads/Gardens/Feoda in a game-ending mega-turn. But mass trashing into Duchies will be difficult.5
Imp has a few other uses besides trashing into VP. In situations where +Buy is more valuable early in the game, such as with Peddlers, or with other Potion costs, you might want a "market" now, but a different 5 later. You might want to trash them late-game into activated Cities. The card offers some intriguing flex potential on a variety of boards. And of course, there's also those other boards where all you really want is +buy or another non-terminal, and it's cost barrier is nothing more than a giant hassle. Those are fun, too.
Anyway, those are my thoughts on my own card. Thanks to those who voted for it and who said nice things about it!
1 If not an actual need.
2 Obviously you'd almost never buy it for 5P.
3 Though an Imp grabbed opportunely can help you win the Scrying Pool split.
4 Say your opponent has trashed down and then re-bloated their deck.
5 Though not impossible, since Imp can still be trashed into Duchy even if there's already one Action Card in play. It self-trashes, so it no longer counts as an "action card in play" when the gaining happens. Scrying Pool or Tactician can help you trash a bunch of Imps into 5s on the same turn.