I had a skin on a vb 3.6 forum once and it always caused me problems. It was always breaking at upgrade time causing me no end of hassle. Though when it worked it looked good. Since then I've always gone for the vanilla-as-it-comes forum with no skins, maybe just tweek the colours. But I've seen a few xF skins recently that look very smart.
what's your experience of third party skins and xF, or don't you bother?
I use a highly-customized skin, it uses a postbit design that was purposefully designed to be similar to my previous vbulletin postbit design
However, it isn't much more than some code in the extra.css template and some values in the style settings. Several upgrades have haven't been difficult, just an occasional template merge. I have reverted a couple minor templates when the opportunity arose to do something better.
Yeh if you do it correctly as many in this thread do, its as easy as upload files, upload .xml, check for anything new that might interfere with your old changes (and if you didn't make changes, then skip that step )