Agree with the rats = tribbles comment. In fact, I recall on the worst card ideas thread someone proposed a tribbles card that was damn similar to rats, in that it self propagated like crazy.
I have to say, without the other cards in this set, it'd be hard to say rats are any good. +1 card just isn't a good enough trash for benefit to play the card with just a plain old trasher, I reckon. Even if you start using trash-for-benefit like Bishop and Apprentice and Trader, I still think you're slowing your tempo too much by committing to rats.
As for mixing it in with cards that have inbuilt trash for benefit, I'm still dubious. Gain the one-shot golds? You've still got to get them in your hand in a deck thats mostly rats. Gain an attack from Squire? Ok, you're attacking now, but you've still slowed yourself more than your opponent. Trash a Feodum? You've spent two $4 buys on gaining three silvers. Trash a Squire to gain a Cultist and then trash the Cultist to draw... more Rats? While you're messing around with this, three piles run out, and your opponent wins on estates.
No, so far, Rats is just a trap, as far as I can tell. Its a worse card than Scout.
Despite being 99% sure, however, there's a nagging voice at the back of my head telling me that Develop / Rats would be awesome...
No, no... it wouldn't.
But would it?