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