I wrote a very long response that got wiped out by accident when i refreshed the page, so heres the short version:
FlexRAID is good for media storage that's not frequently updated. Write speed: Speed of the raw disk being written to. Read speed: Same.
Parity is calculated on a scheduled basis as opposed to doing it as the data is written to disk.
Ultimately the word flexibility is "why use this" as opposed to storage spaces/hardware RAID - as many drives of as many sizes as I want, without wasting anything more than one full disk worth of parity data.
If you're working with constantly changing data, take this off the list.