I have a Promise Fastrack 100, Arco Duplidisk and Duplidisk II and a 3ware 6410.
They all work. I don't really have any personal basis to compare performance other than other people's reviews.
The Promise (in my Win2K desktop) wins on price, certainly, but will only allow 1 mirrored pair, I think. Since it is driver-dependent (and I had some trust issues), I used it to mirror data drives, but I keep "C:"/system on a smaller separate drive which I mirror occasionally with Cheap RAID (Ghost to a backup).
The Arco Duplidisks win on OS transparency. No drivers needed. RAID-1 only. If you break the mirror, drives behave as if they aren't even aware that they were in a mirrored pair. Straightforward, but they have had some problems with Via chipsets.
3ware had announced EOL of all their Escalade single controller boards in favor of their big RAID boxes. They have since partially retracted EOL announcement, but 6000 series are no longer available, just the more expensive 7000 series. *But*, you can still find some 6000 series in the channel. I got a 4-port 6410
here and it works great. The best of my bunch, IMO. RAID 0, 1, 5, 10 and JBOD capability for $99 with Win and Linux drivers. I'm probably going to arder another while they have them.
(edit: the 6410 is UDMA 66 only. Hypermicro shows the ATA100-capable 7410 at $235.)