Under firefox there could be some issues, but under Mobile Safari(and all other WebKit browsers) & desktop Opera it works ok. It is not optimized for mobile devices yet, but zoom in/out is so simple that it is not a real issue.
[Edit]
small tip for iOS users: Instead of original browser I suggest to use any other browser with fullscreen option from AppStore (for example iCab Mobile, Atomic, Mercury, PerfectBrowser etc, etc.). Safari has little bug with rendering this site - stones on the borders of the board are cut off and there is no chat available in horizontal view. Probably it will be corrected by the developer of EGS soon - but till then I suggest to use any other browser.
[edit#2] screenshots from iCabMobile on iPhone4 added
Of course zoom level is scalable so you can have perfect view for every part of the board
Runs under Firefox with no obvious problems so far. But seems rather slow.