I am presently using FMProAdv 10.5 on Snow Leopard. It works as I want it to, and I have no real need to move to 12.
All of my solutions are for my own use. The most complicated it gets is generating. emailing and uploading PDF invoices and generating and uploading HTML files. Emailing is through the 360Works plugin, uploading is through cURL (though I could sync using Transmit if necessary), and there is some minor file manipulation through Troi File.
The OS is a different story. It is getting to be about time I upgraded to Mountain Lion so I can use some newer apps, and I don't want to lose FileMaker in the process.
Can anyone tell me about real world experience running 10.5 on Mountain Lion?