I have yet to hear of anyone using anything except hard drives for backups.
First, there's cost. A decent sized backup can be obtained in most areas for $100 (US) or less. Second, there's the "unknown" nature of SSDs in terms of true reliability. As far as I'm concerned there are still too many "gotcha's" associated with them - like just dropping blocks in the middle of a file.
Some commercial drives are now coming with their own bizarre manufacturer configurations. WD, for example, has a few drives that disable some of the drive features, like the power switch, unless you load their drivers. I believe that when Mavericks came out these drivers were incompatible with the OS and actually caused users to lose data, and I think in some cases delete volumes (sort of kills the purpose of a backup drive.)
Seagate had, or has, some bizarre NTFS formatted drive with what I can only call an "NTFS to HFS" translator. It sounds like a nightmare just waiting to happen to me.
My advice, FWIW, is just buy a case and get a hard drive and build your own. I've never had problems with that.