The first thing to point out is that this was done on an old EEE PC T91MT (the Eee PC with the touch screen). This only just matched the minimum spec for the preview of Win8, so I can forgive the flickering screen and the generally lackluster performance. The Eee's touch screen is also not great, which probably added a low level of background frustration.
The main problem was that I found it really hard to discover where the functionality was. The Charm-bar wasn't easy to get access to for some reason ("NO! I don't want to scroll, I want the Charms!") and often when I started typing on the Start screen, the search wouldn't come up so I had to try and get the Charms up to select search. This made it tedious to find functionality.
I used the Win7 upgrade path, so the PC still had logins for admin (with a password) and logins for family members (without passwords because my kids wouldn't be able to remember them).
In Win7, when you start up the machine you get presented with the list of users to select who you want to log in as. When you restart Win8 it seems to remember the last person logged in, and if they don't have a password, it logs them straight in. This means that if I was using the PC last, but my son fires it up, he gets automatically logged in as me. For a shared family PC this is going to get annoying pretty fast.
Finally, the reason that I have second thoughts about upgrading to Win8 was that I had to search the web to find out how to switch the bloody thing off! This is symptomatic of the whole UI, I feel. There's no way anyone could naturally figure out that you need to to get the Charms up, select settings, then Power and then select Shut Down. Ludicrous usability. With Win7 you see the 'Shut Down' button as soon as you discover the start menu (which is easy to find).
Still going to play with it for a while, and I'll report back on how the children get on with it (aged 8 and 6, with only limited exposure to Win7).