I have been using unRAID for 4 years now, came from RAID 5 (6) 1.5TB Seagate 7200.11 (motherboard - XFX - which died 3 months later - thank God I moved out of hw raid) After moving to unRAID I upgraded to 2TB drives and added more drives.
I have never used FlexRaid (or SnapRaid) so I can't comment on those, but as far as unRAID goes, I have had 0 problems, and I actually host a lot of services on my unRAID box. I have Plex, AirVideo, Twonky, Subsonic, Sick Beard (tv shows search), CouchPotato (movies search), Headphones (msuic search), Transmission (torrents dl), SABnzbd (newsgroup dl), FTP, SSH, heyu & x10commander (X10 home automation), HP laser printer shared and probably other services I forgot to mention.
My current setup:
System: ASRock - 880GM-LE
CPU: AMD PhenomTM II X4 905e - 2.5 GHz
Cache: 512 kB
Memory: 8704 MB - 667 MHz
Network: 1000Mb/s - Full Duplex
Drives: (12) 2TB drives (WDEARX) DATA & parity drives, and (1) 500GB drive (WD5003ABYX) cache drive.
I like unRAID for many reasons, I can host all the services I need, it doesn't stripe my data, I like the fact that it boots off of a USB stick - which I have 2 exact copies and I backup every month or so (and even if I lost both the working USB and the backup one, I can still read my data using reiserFS), and all the drives except the cache (where I host all the services - reason why I chose enterprise drive) drive go to sleep when not in use, extending the life of the drives and energy efficient.
You're setting up a server, not a workstation, correct?