RAID-5 in software would be a nightmare. The parity calculations would suck performance from your machine. While reasonably modern systems could probably handle it, your CPU usage would still spike during disk writes. The faster the disks, the higher your CPU usage.

I know it's a bit of a 'waste', but RAID 0+1 (aka RAID 10,) is much less of a CPU hog, for even better protection.