Have not reflashed after 5 months of heavy use. Runs totally stable and just as fast as out of the box. Maemo is real linux, so this is exactly what I expected. I don't expect to ever need to reflash as long as I run Maemo 5 on the device.

However, since I depend on my N900 as both my only phone (no land line) and for heavy business use, I've been very careful and conservative about what I install and what mods I do.