I've started doing everything in LaTeX - diary, everything I'm learning about, books I'm writing, maths, documenting my programming etc - the last few months too; it's going great. It took a few months to learn about LaTeX packages, basic TeX etc. (And every time I use TikZ I've forgotten it all again..) It's especially useful mathematically in avoiding errors when doing page-long calculations with pen and paper - copying lines instead of writing them out, and the whole thing looking so lovely and neat - eliminates 95% of the errors I made before, and it's faster. I still do ideas, sketches etc on paper, but anything that's likely to be want to be kept goes straight into LaTeX.

But also..all my favourite books have markings on each page, margin comments, turned-over corners etc. Any pdf book I read more than once or twice, I'll get a paper copy, I think.