I bought a Nexus7 specifically to mess about with Ubuntu on. Agreed it's a fair ways off being really usable right now but you can make some headway. Adding an OTG cable and dangling a USB hub with keyboard and mouse off it helps a lot. The you can switch the WM to be, say, metacity, which is a lot more zippy than unity right now.
Once you've done that it's a vaguely reasonable little machine. Still, not something you'd give to your granny but fun for a hack about on.