I think lyx is a great tool to write documents more than 10 pages long. MS Word can get very tricky with figure handling. A couple of years ago when I was writing my PhD thesis, I started off with MS Word and then switched to Lyx because of the hassles word was presenting. I was very satisfied with the quality of the document: http://v.hegadekatte.googlepages.com/Hegadekatte_2006_PhD-Thesis.pdf. I have hence encouraged my students to use lyx and all have been happy with their switch.