I use RaidFrame on an OpenBSD box and am quite happy with it. Like any software RAID solution, the pitfall is that, if/when disaster strikes, you really need to know the right commands to rebuild everything properly. When you're sitting there with important data on the single functional disk of a two disk mirror, you can't afford a typo. Hardware solutions are generally a lot simpler to deal with. I would say that if you're deploying several machines software RAID becomes a better solution, both because the cost savings add up and because you're getting more familiar with the commands and tools. For a single machine that won't be maintained very often, especially if someone else will be maintaining it, a hardware solution would be well worth the money even for a simple two disk mirror.