The Zumo route auto-recalculate is not a problem, it's just different. How it's handled is covered in this thread:
http://www.advrider.com/forums/showthread.php?t=188868&page=3Zumo uses the same City Navigator v8 map detail as the 2820 so all the same map detail is there.
The POI capacity of both units is the same.
Zumo holds 500 waypoints; 2820, 2000. Anyone having more than 500 waypoints should really consider converting them to a POI file as it is very impractical to try to manage that many waypoints on a Garmin unit.
The Zumo is not perfect, no GPS is, but it offers some nice features for bike riders. Some things I especially like are: it's internal battery allowing you to look at routes, POI's etc. in a restaurant or other location without plugging in to a wall outlet. Unique track log features allowing you to easily create routes from track logs (you can also look at your track logs on Google Earth maps). SiRF chipset for faster start up time and less signal loss. It even picks up a full signal inside my house so there should be less signal loss under tree canopies, canyons and around tall buildings. You can operate it wearing gloves. SD card slot. Screen brightness and clarity is very good.
I wouldn't be too influenced by the detour function, it is something that is not used a lot by most. The Zumo has a detour function but is not as sophisticated as the 2820.
The Zumo package is complete with everything needed to mount it on your bike, with the 2820 you'll be buying mounting parts and exploring various mounting hardware options. With either unit you have to purchase the XM receiver/antenna separately.
Here is a 2820 installation thread that will give you some good installation information:
http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=57448 Most of the 2820 hardwire, XM antenna and audio connection information applies to the Zumo too. That thread also includes how to set up the Harley Dealer location POI file with either unit. The thread contains other pertinent info as well.
There have been a lot of reports regarding screen fogging on 2820 units.
I expect Garmin to further enhance Zumo's current features with firmware upgrades like they typically do with a lot of their units.