I've been a fan of this show for many years now. It's online communities are still strong, even though it has been many years since they canceled the show. Check out the yahoo.com groups, there are at least 40 groups devoted to this show. New fan fictions continue to be uploaded. :)
As far as the relationships in the show go, same thing happens to the men. It seems every other episode has the girl Nick or Derek likes being some evil creature, or posessed by an evil creature! Though I have to admit, Nick fighting a sucubus is always one of my favorite things to watch! ;)
The one major gripe I have with it is the way Scifi handled Kat's involvement with Witchcraft. It was totally inaccurate, and as a Wiccan, I find it comic and almost stupid. Especially in Rachel's "Banishing" of Miranda. It would only have worked if she believed it would, which she didn't! It was purile, and very foolish. It made it seem like all you had to do to work magick was to throw your hands in the air and recite a poem.
Other than that, I love the show, and even though there is no longer any hope for new episodes due to the actors distancing themselves from the fans due to an unfortunate misunderstanding with one of them, it remains one of the best shows I've ever had the pleasure to watch. :)