I did my first upgrade yesterday. I had a Sandisk Extreme 32gb CF card from my old Canon SLR that I used. The mini that I bought on eBay already had it's battery replaced so getting the top and bottom off was pretty easy. Removing that metal C clip was a bit difficult.
The mini restored easily with the CF card. No issues. I installed this one in my '06 Nissan Murano which has a dedicated iPod interface that shows the menus on the center console display.
I bought another one for $14 on eBay that only shows the Apple logo. I am hoping/assuming it's just the hard drive that is bad.