I switched from a mechanical Logitech Mouse to a Microsoft Intellimouse Explorer shortly after it became available here, just because cleaning my mouse every other week sucked that much. I definitely wouldn't buy any mechanical mice again, ever.
Opto-mechanical trackballs seem to be okay, though, because there's just not that much dirt, lint and grime to be collected by the rollers, so the primary concern seems to be roller degradation due to wear, which can be avoided or at least made considerably better by using steel rollers. So opto-mechanical tracking on trackballs doesn't seem as bad as it sounds, yet I'd still want to buy an optical version if it's available. Optical tracking is largely ignorant of any dust and does not deteriorate, ever. It uses about two solid-state devices which either work or fail, one being a quite easy replaceable LED (if the sensor itself fails, you're out of luck, though). Opto-mechanical tracking, on the other hand, uses multiple items for each axis to be tracked: A roller with connected axle and encoding wheel, a LED and phototransistor to form a light barrier and control electronics. Electronics are unlikely to fail, but I suppose the weak point (considering the roller is made of high-durability material such as steel; if it's a plastic roller, it will fail first) is the axle's bearing, which is most likely no dedicated bearing, but jsut plain plastic on plastic.
Quality Opto-mechanical trackballs should still last many years, especially if they're built with quality in mind. I'd recommend getting an optical version of your preferred model if it's available for not much of an added bonus (it shouldn't cost more than 20%; in fact, optical technology should be cheaper to employ due to less complicated mechanical parts being used) and sticking with opto-mechanical models otherwise.
-huha