I just ordered some fine pitch BGA boards from Aisler as prototypes. I'll post my experience when they arrive in a couple weeks.
So far I was impressed that the gerber viewer was able to suss out the correct layer information. I just dropped the same ZIP that I generated from Altium for other pcb vendors.
The PCB preview image was also generated 100% correctly, even correctly distinguishing plated/NPTH and supporting slot milling in the outline layer.
Looks promising so far.