I am a fun of reiserfs. Period.
The only filesystem i had without reiserfs was on my laptop. 20GB of data on ext3.
I wanted to converted to reiserfs without doing a tar create a new filesystem on the disk and untar the data. Since i had a working backup set, i give a try on convertfs (http://tzukanov.narod.ru/convertfs/)
Boot from a CD with builtin kernel support for the filesystems you want to convert from and to.
Build the convertfs utilities on another machine and start the conversion.
Warning: It takes a lot of time. On my laptop (Pentium M 1.3 GHz) took about 2 hours to do the conversion.
Change your fstab and boot in a your new filesystem.
Everythink worked as expected.