Are you running one in your A1200? I am, no real speed gain between using one and the onboard.
FastATA and my 060 on the other hand is a massive increase.
I have a IDEfix Express in my main A1200, and one of those cheap 4-way adapters that AmigaKit sold a few years ago in my other A1200.
The speed gain mainly goes for when large files are being copied, under normal use the difference isn't really that big. If the MaxTransfer value is set to 0x1FE00 or lower on the partitions, the speed will of course not be any faster.