Well, yes. But (unless you're good with LaTeX) try taking notes in any subject using calculus in a text editor.

The sketch program, of course, is really abysmal.You could tell this by the fact that Nokia developed it. Seriously, though, it's a (very rudimentary) raster graphics program -- which is exactly wrong for taking notes. You want vector graphics, and if possible extra stuff like inserting text (to take advantage of the speed of text input for the parts of your notes that don't need geometric layout). Xournal is the best I've used, although liqbase comes close.

So comparing Xournal to some real text editor like joe that might be used for text-only note-taking would be more fair, and I at least find Xournal highly usable. 3.5" screen? Almost the same usability, because I can't write on the whole screen -- I have to rest my hand on the bezel, so I can only effectively write on the right 2/3. With the smaller screen, I'll be able to see less (without zooming) but I'll be able to write essentially the same increment between scrolling the page.

For me (for some types of use*), the 'default' method of use is to have the stylus already between my fingers -- hence no thought-train-breakage. (I tend to use a soft-plastic mechanical pencil, with the lead retracted. Full length makes an unimaginable improvement in stylus utility, which is one reason I'm not too keen on the guitar-pick. I can always hook a mechanical-pencil-stylus on a strap if I want; since it's not in the device, why should it be small?!)

*I don't like speaking up in these things -- afraid it'll turn out like the guy who walked out into a Civil War battlefield with a grey shirt and blue trousers. I use stylus for some things, and fingers for other things, and randomly use whatever I was last using for the majority. I'd hate it for either one to go away, which is why I want a screen with capacitve touch and active digitizer....