I've played to a limited extent with Evernote on both Macintosh and Windows 7.
And certainly on Mac having a native client Evernote beats OneNote hands down, with nice integration to the browser and the ability to save pdfs of 'printed' documents into the notebook.
The user interface is not as slick as OneNote's native windows client, and the need to tag documents consistently needs a little more discipline than perhaps I possess. That said it works well and is a usable product.
I am less convinced about the windows client. Side by side with OneNote it seems clunky and, initially it seemed to lack the browser and printer subsystem integration features of the Mac client, giving it not much more functionality than working via OneNotes's web client.
However, while the Mac install asks you about preferred browsers and offers the Firefox extension, the Windows installation assumes that you of course want to use IE. Well about 30% of windows users, including me, prefer Firefox these days. Adding the Evernote firefox extension allows you to select text on a page and send it direct to Evernote.
There is still, however, no easy way of getting non web documents into Evernote via the printing subsystem as is possible in the Mac environment or with OneNote.
However having a native fat client makes it quicker to use than a web only solution and being multi-platform is a definite plus. Sychronisation between clients, even those on different platforms is unobtrusive and efficient - trul dropbox style sysnchronisation.
Evernote is not sufficiently good to make me want to dump OneNote. Its superior Mac client makes me prefer it for my professional data management work, purely because I tend to do that work on a Mac, and it is incredibly useful to also be able to access and work on that material from home where I normally, but not exclusively use and Windows 7 machine.
Equally I tend to pursue my historical and archaeological interests at home and OneNote's better integration with windows suits me better for that work.
At the moment I'm not using a linux based desktop for anything serious. Consequently, I've not explored Nevernote further. Where that to change having true cross platform integration might indeed prove to be the killer application for me. Until then Nevernote and linux integration remains a potentially useful feature rather than a 'must have'.