Biggest reason for upgrading to 2010 myself is that I'm a student, and OneNote 2010 adds equation support, which is great for math classes. That, plus students get a good discount as well .

OneNote is probably the one application that doesn't have a good equivalent in other office suites.

I used to hate ribbons. After a while I grew to love it, and hate old menu system. Ribbon system is just so much easier once skill and knowledge is up. 2003 I have been using for years and I still have to search and find some features.

I'm was a huge OneNote fan.  But since I moved to the dark side, I use Basket.  http://basket.kde.org/ 

Looks a bit more messy and unfocused than OneNote.

OneNote is very much focused on the idea of being a notebook style application. It has the organization structure of Notebooks, sections, and pages, which works very well for an application designed for taking notes.

OneNote also has drawing tools, handwriting recognition, and in 2010, ink to math. Makes it an excellent tool to use with a tablet PC.  Even with a mouse, I may occasionally draw a diagram with it. If there's an application that could replace a paper notebook and pencil, this could easily be it.

It also has voice and video recording, which takes a bit of drive space but can be very useful for replaying an important presentation.

Basket looks to be Linux-only, and only really available as source code. And it just looks like a UI mess. And it likely lacks a lot of useful OneNote features. This does not look like a decent application to replace OneNote. From the screen shots and description list, I don't really get the "this could replace a paper notebook and pencil" impression with Basket.

I suppose if you're using Linux, you can't really use OneNote.

But Linux - never really made it to the desktop. And to be honest, it still lacks a bit when it comes to applications. Sure, it has a great browser, and a decent word processor/office suite, but after that, it sorta falls off. The other applications for Linux aren't really anything to get excited about.

I probably would have agreed with you on that a few years ago, but: 

It might come down to usage habits.  I really don't have much use for voice recording or handwriting recognition.  The feature that made the switch a no brainer for me:  the ability to sling notes/images/anything with just hot keys.  That and the super karamba theme is pretty pimp as well. 

FWIW:  I'm pretty certain Basket is available in just about every major distro's repository.  But you're right, you do have to step to the darkside to get a hold of the goodness.  And, I do run MSO on a virtualized xp install but I'm pretty sure it runs in WINE as well.