I'm an Ace author.
My experience with them has been very positive -- any caveats i might have are totally minor compared to things I've heard from fellow authors about problems with other houses. Good editors, good design, totally professional staff.
Ace is notorious for being on the low end of advances, but if you have confidence in your ability to earn out, that's not so much of an issue. And on the upside, they also have the reputation of giving their authors a chance to find their audience, which is even more important. They're not going to dump you just because your first book doesn't quite perform up to expectations, the way some others might.
As a mid lister, I don't get a lot of publicity help from them, but they certainly have done at least some. And they're really good at getting your books into stores, including chains, which is the most vital thing, imo.
Their marketing strategy tends toward mmpb originals. If you hit it big, they'll go with hardcover later on. I think it's a good strategy, and in today's book climate it's looking better than ever -- as opposed to Tor, for instance, who tends toward hardcover first, then a mmpb later.
Finally, while other houses are suffering through shakeups during the economic downturn, Penguin, and Ace/Roc in particular are pretty solid. Their books are selling, and editors aren't losing their jobs.