In response to a comment left on a post below, yes it was very windy in Hawai’i which did interfere with the seeing conditions (as did the nearly full moon on that trip). More generally, though, what professionals want from an observing site can be different from amateur requirements. Most modern large optical telescopes allow a breeze to pass through the telescope – you can see the slits designed for just this purpose on this picture of the VLT. This helps to keep seeing steady and the mirrors clean, but of course a strong breeze is severely detrimental to a small telescope with a closed tube, as Damian found out when trying to image from just next to the VLT.
On the summit of Mauna Kea, with the Moon out of the way the sky is stunning. But only when you hyperventilate a little to get enough oxygen into your lungs – otherwise the altitude ensures the sky is somewhat disappointing. The best views I’ve had from Hawai’i have been down at the lodge when you can walk out of a fully lit room and see the Milky Way straight away.
Update : Thanks to Kaustav Bhattachar for pointing out my broken links. It all works now.