A few months ago I published Foo Foo on Smashwords. It took a month of waiting, but it ended up on the Smashwords "Premium Catalog". While this did not generate any extra sales, the extra potential purchase avenues are ones you want to be in.
Since, I have updated Foo Foo three times, correcting errors, and niggling the details as Tolkien would say. The updating process, provided your document conforms to the Smashwords publishing guide, is absolutely painless. While I haven't sold enough books to warrant receipt of a check from Smashwords (yet!), their online store is pretty awesome. It has a coupon manager you can use to provide discounts, which is really useful for promoting your work.
Smashwords also provides ISBN numbers for your books. That service alone is really awesome, and it doesn't cost you a dime.
Overall, as an author, I've been really impressed with Smashwords.