I did an upgrade (Vista64 Ultimate to Win7 64 Ultimate) on my laptop, forgot to unplug the external 250GB HDD which has about 200gig of data (Music and Photos) on it.
I reckon the upgrade took about 50 mins of which about 30 mins seemed to be stuck whilst I presume, due to the thrashing noise, that the drive was being indexed.
On the plus side I haven't had the constant HDD activity light that I got for the first week of Vista so I suppose that half an hour is a small price to pay.
First impressions are:-
No hassles with upgrade, drivers for everything without interweb searching
3 programs were "incompatible", none were vital, Approach had issues apparently with Vista too but I'm not sure what, a Canon program that I should have uninstalled ages ago and some old Intel WiMax thingy that I downloaded by mistake when looking for a wifi driver. Removing the last two would have stopped pop up warnings in Vista so I thought I'd take the hint with Win7.
I like the cleaner Windows Explorer look, I like the way you can split two documents across the screen and I like the pin the icon to the taskbar thingy, it might make my desktop tidier.
I don't like the way that Win7 just doesn't "look" different enough to Vista, I haven't spent this much money for a double height taskbar

Super Pi doesn't crash randomly in win 7 like it does in Vista64 and it runs (slightly) faster too.
Despite the fact that normally I'd clean install a new OS I have to say that so far I've had no problems with the upgrade, admittedly this installation of Vista is relatively recent having gone from 32 to 64 bit Vista only recently on the laptop.