Rooftops are great under perfect conditions, but I have trouble setting up good rain and wind cover on them sometimes -- waking up to my stuff getting soaked is a real deal breaker, and wind in my face all night gives me the sniffles the next day. I'm also not a great climber, especially with my pack, so I tend to look for ground level accommodation
I can't say the experience is really something I "like", but vacant commercial units in otherwise still-active strip malls tend to be my staple. Too many property managers don't bother to properly secure these empty units, providing easy non-destructive access to a safe place to sleep. I guess this is more a squat then a camp, but I still use my camping gear and tend not to stay more than a night or two. I'm a strict adherent to the leave-no-trace philosophy of urban camping -- I sometimes even clean these spaces while I'm there if they've left a mop behind, or shovel the walk outside when it snows if there's a shovel, so its never really resulted in any problems, even the odd time I've been caught in one. Ideally there should be a back door to escape through in case they drop by before you wake up in the morning (oversleeping is my worst enemy in these), but otherwise I get the best sleep possible in these locations.
If these can't be found nearby wherever I am, I tend to find some shrub or ground cover that just barely covers the critical sight lines in a low-traffic, hard-to-walk-to spot that, if you looked at it, you'd think no one would ever attempt to camp right there in the almost-open. A little improvised camouflage hung in the right spot, a discrete-colored bivvy, arrive and leave while its still dark, and grab a nap later mid-day and everything seems to work out fine. Maybe I've been lucky, but these tend to be my favorite experiences -- one day i wanna sneak into disneyland and crash under a roller coaster or something.