I really like using Smashwords to get my books out to people who use e-readers. It's what I would call the one stop place where you can reach a larger audience. Not only do your books get bought by people directly from the Smashwords site, they also become available at the sites offering the version formatted for their particular reader such as at Amazon and B&N.
Once you have your manuscript typed into your computer, you're almost ready to upload the file. Take your time to read through the style guide and formatting guide at Smashwords and make sure your document meets those guidelines. When you click to upload your file, the computer at Smashwords will automatically format it for all of the e-readers. It's so much easier than having to create different files for different formatting rules at several different sites. Almost as soon as that happens, people can start downloading the book from the site. If there is any problems with the formatting of the original file, you'll receive a message telling you it won't be sent out to the other sites in their catalog and won't be available at some of those places unless you make some changes. That's a summary, but it's really fairly easy.
You do have to have a PayPal account in order to receive your royalties. You get slightly more for each sell if it's downloaded at Smashwords than at the other sites, but I think they it's a good percentage. I get around $1.80 for each book sold from the B&N and other sites and my books are priced at $2.99. You can have your book sent to Amazon as a Kindle, but I usually don't. I upload directly to my Kindle account because I had it first.
So far I've been totally satisfied with the service I've found using Smashwords to sell my e-books.