A couple of weeks ago, my much-loved Logitech mx600 mouse died after a few years of extremely heavy use. So I went on a semi-obsessive research hunt for a replacement that would give me the features I wanted and a decent price.
I learned that the Marathon m705 was probably the best bet in terms of size, features, and price, so I was all geared up to buy one online...and then I saw conflicting reports on whether the thumb button registers in xev or not. Arrgh. More research didn't help matters, so I eventually just went out and picked one up at Staples to find out firsthand; if it didn't work, I'd return it.
Answer: yes, it works perfectly, the thumb button registers in xev as button 10 (same as on the Performance MX). I'm in Debian Squeeze, but that shouldn't have anything to do with it. Hopefully noting this up on the web will save someone else frustration & research time (I've seen my posts show up on Google in similar circumstances before).