I don't know what device you are on, but current android devices even support HTML5 fullscreen api, which makes viewing the slides a amazing experience!
Yeah, used it before. It's a great service, don't get me wrong! What annoyed me the most about speakerdeck.com is that it is absolutely not usable on mobile. And it even can't because it uses it's own embed player (which is a good idea, in general [dogfooding]). But the embed player uses a iframe, and getting responsive with iframes is at least quite a good challenge.
Also that slidr.io uses pdf.js for the slide rendering, with that there is no slide processing necessary anymore, that was another fact that annoyed me at speakerdeck. After uploading quite a big deck I had to wait hours. slidr.io just delivers the pdf to the user to it is usable right after you are done uploading. With slidr.io using pdf.js there comes tons of other great things, for example there is a actual textlayer above the slides. So you can finally click links or copy (and tweet) texts, also the accessibility improves a lot because screen readers can read the slides.
Hope that gives you quite a good idea what drove me to create slidr.io :)