@webcomictalk - I've done two crossovers in my Closetspace comic, one major, one minor. The minor one was simply having an angel character from my A Wish for Wings Comic appear in my Closetspce Comic.
The major one, though, was a crossover with Venus Envy. This was planned between me and VE's creator Erin Lindsey/Crystal Frasier to be one where we both worked on each others comics, however, it ended up only being my side of the crossover getting done (although with her permission and supervision of her characters).
What was important, though, was that the corssover HAD to do two things: (1) it it had to further my story and (2) my main story had to make sense WITHOUT the crossover in case I ended up not being able to reprint the VE characters in book form. I think I managed on both counts, and while the story makes sense without the crossover, it makes better sense with it. But that's what's important.
Having the crossover is great and all, but in the end, it has to service the story and not just be a crossover for crossover's sake.
I never did find out what Erin/Crystal had in mind for my her character living in my character's world, other than she got a tattoo on her arm that said "Mario" and was wearing a bee costume by the end of her side of the adventure.