I've been playing since it was released on Windows Phone and I'm currently up to 56 levels and 79 bitizens.
I usually just pop it open first thing each morning, take care of business and then forget about it the rest of the day. By now I'm able to earn enough credits to add a new level every day; however, I'll have to admit that it's become quite laggy. I'm running it on a L925.
I spend my bux on elevator upgrades. I'm on my 3rd elevator, but the lagginess has slowed it down considerably.
Any tips or hints from anyone else out there?