As someone who is familiar with Windows 8 SS (such a bad choice of names, with that acronym) and FlexRAID (and has used DriveBender, too!), they really address different groups.
FlexRAID is for people who would have a huge shiny enterprise level RAID solution, but can't afford it or can't justify the cost. It's not that easy to set up, but it does a fine job and is largely set and forget. The parity and mixed-drive-size options are robust.
Windows 8 Storage Spaces is for people who can't remember if the new Bieber album they downloaded is on D: or E:, and have no backups nor any idea what parity is.
Both do a job well, there is some overlap, but the focus is very different.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.