Since I put Arrival up, I've had a few questions about Smashwords, most asking about how easy it is to use. As I only have one story up, there are better people to ask, but this is just my opinion.
It's pretty easy to use, taking a standard .doc Word file. No real technical knowledge is required. As long as you make sure that you follow the style guide, paying attention to things like paragraph formatting and breaks the results are pretty good. It took a couple of hours to get Arrival into shape, but if you aren't aiming for the Premium catalog, you can be less precise. However there are a few snags that came up which it didn't cover.
Some characters don't go through the conversion process e.g. I lost my en-dash character. Also, smart quotes and apostrophes appear OK on most formats but play havoc with .txt. If you don't want .txt it's not an issue, but if you do, make sure you've removed them all. They offer a very good range of formats including .epub and .mobi through to .txt and .html.
Overall though it's quick, fairly simple, and free. I used it for a giveaway, so I don't know how good it would be for commercial publishing.
Once it is up you can submit the book to their premium catalog, which puts it in places like Kobo and Amazon's Kindle store. To be accepted you have to make sure the formatting is correct or it will be rejected. They will also assign an ISBN for some stores, so you will have to opt out of these if you don't want the book to have one.
The analytics ae fairly simple, showing how many people have downloaded the book and how many people viewed the book's homepage per day. I'd have preferred slightly more detail, e.g. download format, but there is enough there to tell what effect promotions have on downloads which is useful.
Overall I'd have to say it's easy, free and reaches a wide audience. For promotional purposes at least, Smashwords is very useful.