Just to back up xnappo's experience, I'm also using the Celeron NUC as my HTPC running Openelec off a thumb drive (16GB SanDisk Cruzer Fit) and I haven't had any issues with slow navigation or response time either. I've never timed how long it takes to boot because I just wake it from suspend using my Harmony 650 remote + the USB IR receiver the Rosewill RHRC-1102 remote (fits perfectly in the NUC's front USB port and was plug and play from the start).
As for the fanart and other metadata, some of it gets downloaded to the NUC's USB drive but if you want to save extra fanart using an addon, etc. you can save all that with your media files so it's not taking extra space on your thumb drive. The addon I used with the ACE skin to download additional fanart saved it to those files on my server automatically.
Do you recall what type of memory you put in it and how much? I was looking at all the DDR3/1333/1600 options on Newegg but wasn't sure what to go with.