My prefered UMPC is a modified Sony UX280p (as Sony discontinued the product line... I have found a very bright guy who continues the development of the product alone... he installed a core2duo (U7700) and a 120gb hdd in my UX... making it the most powerfull UMPC ever made...).
With this device, I have always a versatile it system with me wherever I go, and when docked, its an average desktop PC able to cover most needs (except high end gaming...). So there is no need to synchronize anything as I have the core of my main computer with me anywhere...
It has:
-U7700@1.33ghz (but can be adjusted from 485mhz to 1450mhz)
-weak 950gma (adjusted 200mhz to 400mhz)
-Touchscreen (if you haven't tried firefox with the grab&drag module and a touchscreen... you can't know what having a clever interface means...

you can also try cooliris... or play with crayon physics...etc.)
-trackpoint stuff
-fingerprint reader to avoid entering manualy all your passwords
-sliding screen which reveals a little backlighted keyboard (not a productivity tool, but usable...)
-edge modem, enough for internet/youtube/world of warcraft/eve-online/skype/etc. too weak for webtvs
-3 to 6 hours on battery, depending mostly on cpu workload and radios (wifi/bt2/wwan)
-it can run xp/vista/w7/ubuntu/mac osX
-two cameras
+I have a 3.5gusb modem when I need it, a bt gps, a car charger, usb-dvd/rw, usb 1 to hdd
Then, has I always need to have a real computer with me anywhere, but hate having a suitcase for a laptop... I couldn't live without this device anymore....
(By the way, the guy I was speaking about should soon prepare for me a UX490/core2duo/128GB SSD/XP/vista/MacosX... which should be fun too...)