I'm having all sorts of trouble with my ACA620, design-related.
The card will snap out of the CPU with the slightest bang or vibration to the computer. I know this changes from A600 to A600 but it is a little bit annoying. I am afraid of screwing it in place, as no instructions for this are provided officially (specially in terms of what we're supposed to buy) and my fear of the card bending if it still snaps out of the CPU while screwed onto the HD enclosure holes. (ideas?)
The card is also prone to overheating on really hot conditions (ie: usually not in Europe :P). A room temperature of ~30 degrees will bring problems after a while.
Love the unit and the cheap price, but unfortunately it has turned my A600 in an unreliable machine I can't trust when I am on the road playing gigs.