We get possums around here all the time. Like everyone else said, they are fairly harmless and won't bother the cats. From time to time, a mom possum will leave her kids in my covered patio while she goes off to do possum things. I normally can tell when because the cats are suddenly very interested in nocks between pots or the babies try to get out of curious paw reach by climbing up the taller plants. I probably physically see one possum every other year (excluding the kids I am apparently babysitting for) but I can usually see one a week on my security cameras.