Upsides? Downsides? Would you recommend it, or do you have any other e-publisher you would recommend?
Whether Smash is a good choice for you or not depends on your goals.
When you upload your masterpiece there, you can choose in which formats you want it to be available. Might as well leave all of them checked--you never known what someone will want.
You can also choose where you want it distributed. This is turned on everywhere by default. You go into the Channel manager to opt-out of the places you don't want distribution.
Smashwords used to be the big cheese as far as distribution and a simple way for people not in the US to get their books to Barnes and Noble, where you must be a US citizen to upload directly.
You also have the option with Smashwords to make your ebook free on the Smash website, where people can buy it directly, as well as free at B&N, Kobo and Apple (and perhaps other places like Sony and Diesel, but I'm not sure about that). A freebie at B&N, Kobo and Apple can not only raise your sales at those places (provided you have more titles available for people to buy, and they're appealing to the people downloading the freebie), but can also prompt Amazon to price-match the title to free in their store, which can really drive Amazon sales if, again, you have other appealing titles.
Some serious drawbacks to using Smashwords, however, are that it often takes weeks for a title you distribute through them to show up elsewhere, if it ever does. And if you want to change the price, you know, good luck on that happening speedily, if at all. If you want to remove a book, you can have the same problem. You get reporting once a month if you're lucky--lately, B&N numbers have been held up a couple months at a time or more, frustrating those who must rely on a distributor to get into B&N. And in a recent not-fun moment for many people, Apple's numbers updated the day after the payment cutoff, leaving hundreds and sometimes thousands of dollars to sit for another quarter, when by all rights it should have been paid out. I don't sell enough on Apple for that make me want to commit a felony, but I know some who had an extra bourbon before bed that day.
There's also the matter of Smashwords' customer service, and by matter I mean lack. Sometimes you'll hit it on the right day with the right question, and all is well. Other times it can take a few weeks for a rude response that doesn't nothing to help. Sometimes, questions go unanswered altogether.
Another distributor now does the things Smashwords does, and I'd urge you to consider them. (I am not an owner or anyone with a financial stake in them, FYI, just an impressed user.) Draft2digital.com distributes to B&N, Apple, Amazon and a place or two I can't remember, because I don't care about them. Oh, Kobo, maybe one other place.
While the wait for you book to show up at B&N through Smash is typically about 3 weeks, often it's hours to a day or two through D2D. They also pay monthly, not quarterly, and you get actual reporting so you know how you're selling, rather than waiting for one report every month or two. This can be helpful for promotions and such.
The delay getting to Apple can be weeks, but that's Apple. You can go free on Apple (and I think Kobo) through D2D, but not Barnes and Noble, unfortunately. This is a drawback if you use them to distribute there and are hoping for Amazon to price match. They either won't if you're charging at B&N, or it'll bounce back to paid when their bots detect it. Not that price-matching to free is ever guaranteed, but it's unwise to have it not-free anywhere while trying this.
People got paid on-time during the first payment round with D2D. Their customer service is fast and helpful so far. And if someone like Apple rejects your book, they tell you, unlike Smashwords. It's improving all the time, and definitely something to consider. Weigh your options--Smashwords isn't the only game in town anymore.
I would, however, heartily recommend that you go direct wherever you can. Yes, you can distribute to Amazon (and CreateSpace print publishing even) with D2D. But why pay them to do what you can do on your own? I never sold a single copy of anything at Kobo through Smashwords. I started selling there when I went direct (probably because I categorized the titles correctly and didn't mangle the blurbs the way Smash sometimes can), and now I've made several hundred dollars through a retailer that I couldn't break into before. If you're in the US, go through PubIt for B&N, Writing Life for Kobo and KDP for Amazon. Apple is more complicated, and may not be worth the money or aggravation to go direct.
Even if you choose D2D for your distribution (and that's all they do--no D2D store), you can still publish at Smashwords and just opt out of the other stores. I do this and make a nice quarterly check just from sales at Smash proper.
I hope some of this helps you make a decision.