I have a Zumo XT 5.5"
I'm a fairly recent Zumo owner, but long time Garmin user (60Cx & Montana 610, plus several Nuvis) and so far have limited use on the bike. I have done one dual sport ride with it but was primarily tracking, not routing. I've done 1 long day's street ride routing.
I did do a 10 day 8 state road trip by car and used it on that trip essentially to practice with it. I created each day's route on the laptop and downloaded it to the Zumo then followed it. I wanted to see how accurate it was and how many times it hiccup'd. I'd say overall it was a great success with only one day in the Black Hills following a very complex route (kept increasing route complexity to test it) where it got confused and failed to route properly. But that route had several backtracks, figure 8 sections etc., so it was complex. So what I learned was if it's really complex, it's better to break the route into sections.
The interface and touchscreen are a joy to use compared to the Montana 610, which was a huge upgrade over the 60Cx. It's finally responsive, the display looks modern, and has decent storage capacity. For once I'm not trying to parse out exactly how to get all the maps on it I want, just load 'em up. I have a 128GB MicroSD card in mine and I think the max is 256GB, which honestly is overkill. My 128 card is less than 1/3 full with all those Topo maps etc. loaded.
I have not jumped into the whole Bluetooth feature set yet, need to do that as I have a Cardo Packtalk unit.
I have successfully loaded the older 24k Topo maps on it but have found they don't look as good on it as on the earlier units, but they work. I have also loaded the OSM Topo maps on it but so far I don't see too much improvement over the stock maps which are essentially City Navigator with a Topo overlay that's based on the OSM maps. I haven't tried more custom maps like the California Trail Maps.
I have found out the unit runs on Android vs Garmin's proprietary OS like the older units, probably why it works like a modern unit.
The one thing I'm not so happy about is the mounting. The retention "lip" on the cradle is dang tiny and you have to be very certain you've got it locked in securely before riding off. As dirt builds up on the sliding locking piece it doesn't always want to snap in 100% like it should so be aware. I much prefer the Montana mounting cradle, it's vastly more robust, reliable, and secure.
I kluged up a car mount using some parts from a Nuvi bean bag style mount. I actually liked how it worked in the car well enough to decide to buy a spare Nuvi mount kit and build a car bean bag type mount for it so I could put the other mount back where it came from.
Overall I'd highly recommend it. I was fortunate to be able to get mine while Garmin was running a $100 off sale that eased the purchase pain a great deal. I had stayed away from previous Zumo units based on cost.
The only thing I've read about as a problem is some units seem to not work properly in the rain. I think Garmin knows about it and from what I read they warrantied those units. I have not tried mine in the rain because it seems like rain has been banned in California, at least SoCal.
I attached as shot of it mounted on my Urban GS for size reference. Also, it's the same width as my Montana 610 but 1/2" taller, yet offers up something like 40% more screen area.