I've recently picked up a Patriot Memory Convoy XL for my own personal use; partly to for myself and partly to test for use in the company I work for. For a slight premium, we can fit 2 2.5" drives with hardware raid, in the same form factor as a 3.5" drive with a SATA connection. Secondly it can be used via USB.
I personally have been using at via USB for a number of weeks and love it. Having slightly higher redundancy on a USB working drive brings a sense of comfort to me. I like it enough that I haven't been using my firewire drives, I can accept the performance hit of the USB vs firewire.
At the office we are contemplating replacing the single hard drive in a couple of critical work stations with a RAID solution. There are always a handful of workstations that downtime is only nominally acceptable; data loss due to hardware failure is marginally tolerated, and regardless lectures about best practices, and storing stuff on the network file servers... well we know the storey. This doesn't prevent file deletion, but keeps the machine hobbling through a single drive failure.
I haven't ran the disks in degraded mode yet but I will and provided an addenda to this note.
Patriot also has a model that puts 4 2.5" drives in the for factor of a 5.25" drive (about the size of a cdrom drive), that looks interesting, but currently I don't have a pressing need for such an animal. I could foresee using it in some small form factor applications though.