I have a Sil3132 one from scan in the UK - it looks identical to the Sonnet card and although you can't boot OS X from it, you can boot Windows (if you have a boot camp partition on your internal HDD).
I've got a nice IcyBox st266UB USB/eSATA 2.5" drive enclosure hanging off it, you can power it from USB whilst using eSATA for data, which is a nice touch.