I'm pretty happy with the HP 5101. With 2GB of memory and having a ssd, it is pretty zippy. Windows 7 Ultimate installs just fine and the keyboard is large enough that you aren't trying to jam your hands together to type.
However, the high resolution screen really makes the difference. Having the extra 168 pixels (1366x768) down helps keep dialog boxes and other things without scrollbars from getting cut off by the bottom of the screen.
It is very straightforward to replace one of the DIMMs. You just pop the battery out and push further and a panel opens on the bottom where the DIMM can be added. So, if HP is charging way too much for memory, going third party doesn't involve taking apart the machine.
It was a little disappointing the screen isn't bigger. There seems to be a lot of wasted space which could be used by a bigger LCD.
The system is also very smudgy. The trackpad, the shiny black case, etc. A fingerprint reader would also be a nice touch.
Finally, the power brick has an oddness to it. The plug that goes into the unit seems way too long. Since the power comes out the side, this creates a need for extra room by the left side.
Nevertheless, this is a big step forward from a Dell mini9. While the mini9 is a little lighter, the HP 5101 is more usable.
So out of the two generations of the Samsung Q1U UMPCs that I've had and the Dell mini9, the HP is the most useful unit. (The Samsung units were nice but did fall in the category of being either too big or being too small.)
(Edited to fix memory. Should have been 2GB and not 4GB. Thanks Gary)