I don't believe I've ever read a true crossover (excluding Ats/BtVS clearly). And after reading this, I don't think I will ever want to ever :P

There are actually some quite good ones out there - if you know where to look. I generally rely on people's recs.
(My favourites being a BtVS/Harry Potter one, a BtVS/Law&Order, a BtVS/Stargate, a Stargate/Farscape, a BtVS/Doctor Who, and a BtVS/Sesame Street.)

Also, how do you feel about crossover fan vids?

They're fun! Especially that Buffy Vs Edward one that's been floating round the web lately...