For the unfamiliar, Om is a ClojureScript React wrapper that makes React much nicer to work with and adds some nice performance benefits out-of-the-box. Nolan's intro post is pretty inspiring: http://swannodette.github.io/2013/12/17/the-future-of-javasc...
I'm also wondering, are there any difficulties that arise when working with canvas and svg using just Om, or in plain React?
I can't say much about canvas, but using svg with React is a bit of a hassle. There isn't built-in support for every svg tag and attribute, so we have to maintain our own fork: https://github.com/precursorapp/react
Once you add support for the tags, though, it's no different than working with any other html in React. I think we'll see React start to eat into d3's market-share pretty soon.
(defcomponent circle-view [data]
(render [_]
(let [size (get-in data [:circles :size])]
(html [:div.circle
[:svg {:height size
:width size}
[:circle {:cx (/ size 2)
:cy (/ size 2)
:fill "yellow"
:r (/ size 2)
:stroke "green"
:stroke-width 4}]]]))))
Some of the things we had to add for Precursor were clipPath, foreginObject, mask, patternTransform, marker, and vector-effect.