I'm gonna leave superficial details (e.g., tuners, finish, etc.) out of this. First and foremost: It's mine! I've got a freakin' Casino! Given the history associated with that particular model instrument, the mojo factor alone is through the roof. What's not to love about that?!
Secondly, it's got a wonderful tone. P-90s, I find, have the right blend of warmth and bite; the best of single coil and humbucker tones. The first time I played "I Want You (She's So Heavy)" on it, I nearly had an orgasm right then and there. Not only did it sound like the record (as it should have), but when I switched to the neck pickup for the middle solo and drove it, it sang with such a sensual tone that set my soul ablaze.
Thirdly, it's perfectly usable unplugged. When I'm practicing without an amp, I almost always go for the Casino. Sure, it sounds different from my acoustic, but I find it easier to play (probably because its body is thinner) while, at the same time, maintaining the richness of, say, an open G chord at a decent volume.
Lastly, and most importantly, it just feels right in my hands. Sure, my Les Paul's neck is slimmer and, probably, better suited for my relatively small hands, but there's a great balance of the thickness of the neck profile and the width of the neck that I find enjoyable and unnoticeable when I play. I think I covered all the bases.