One of the main problems with the Zumo over the 60c range is that if you have uploaded a route in the Zumo and you aren't starting from the beginning of the route it will keep trying to send you back to the beginning of the route to start the route from the beginning. Fine if that's what you want to do but with the 60C range you can easily manipulate the route by inserting via points and taking off and following the route from where you intersect it. The 60c range is designed for outdoor activities and it does turn by turn road navigation as well, The Zumo is designed for turn by turn navigation and doesn't do the hiking , trekking, off road stuff as well as the 60C range.
If you want a go anywhere , do anything kind of unit that doesn't have a sexy female voice , MP3 player, blue tooth phone connection and other gadgets then the 60C range are very fine capable all round GPS navigation units. If you want the whilstles and bells and aren't likely to do much off piste dirty navigating the the Zumo is an excellent bit of kit. I have a friend who uses one for road trips but any time we go off road , out comes his old GPSV with 19mb of memory because it's much better for non-road based navigating and track following than the road biased Zumo.
I have a 60C myself with 56mb of memory and it's brilliant on the bike. It's well made, waterproof and does more than I need - of course I'd like more internal memory but it's never been a real problem yet travelling around Europe.

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I bought a 60CSX for doing the Stella Alpina this summer , and also for many of the reasons quoted above. My road device is a Quest I and the high altittude seemed to cause it minor niggles ( ages to cold and warm start, condensation on the screen ) but the CSX never missed a beat.

Having the ability to store 5000 waypoints helped as well also the exchangable memory cards.

I used the Quest for road navigation and the CSX for storing usefull data ( BMW Dealers, F1 Hotels, and Michelin Campsite list) and for checking the barometer and altittude. The electronic compass is really cool as well. It came into it's own on the Colle tho' as it is far more rugged and less likely to damage than the Quest and similar models.

The SirfIII chip far out performs the old chips and is a worthwhile upgrade.

Saying that , since my Quest has been home and dried out it it is still a good road tool and with GB Topo maps loaded you can even geocache with it But can't justify two GPS's so Quest and Ram Mount/ Qpac will be up for sale shortly.