A RAID 5 NAS can provide everyone in your home with a simple way to back up and move large amounts of data. My household's NAS is a critical piece of infrastructure to support the evolution of self-expression in schools. Where I wrote papers by hand for middle school and high school classes, my sons cut CDs, use desktop publishing, and create movies - using the NAS to move iMovie and iDVD projects from computer to computer. And, because the kids forget about the projects and leave them on the server, an archive of their creativity is automatically made. (I also secretly horde my kids' art projects in paper form!)
So should you buy or build a RAID 5 NAS? The better question is, "why should I put off moving my data to RAID 5?" As we acquire gigabytes (and in some cases Terabytes) of valuable content in digital form, the robustness of that data becomes an increasingly important issue. Anyone who has had his or her iTunes library wiped out due to a hard drive failure already has felt the sting of relying on a single drive for data storage.
Storage and / or backup on a RAID 5 NAS can provide a step up in keeping your data safe from hard drive failure, but can't protect against fire or theft or an angry teenager out to teach the folks a lesson. So it can be an important part of a data security plan, but not a complete substitute for a comprehensive backup strategy.
The make versus buy decision is likely to be shaped by the size of your pile of old equipment. Many people already own all the hardware needed except the hardware RAID card. Any old "too good to throw out, but not good enough to use" PC is a candidate for becoming a RAID 5 NAS. With an $80 MegaRAID card (and maybe a cheap gigabit NIC) you can turn "ol' dusty" into a very good RAID 5 NAS for a very low price.