The 3900 and it's little brother, the 2960 are superb - rock solid stable.
They do run the Linux OS so the web interface is very different (though I prefer it)
There is a security advisory out of these two (and the 300B) to urgently upgrade to firmware version 1.5.1

I currently have a 2960 handling a 500 Mb/s Virgin Media business cable service with no issues, running at full speed.

The 3910 looked a logical upgrade in the future - but for now I'll steer clear, as it looks like proper testing was lacking in the case of this unit. Hopefully it is fixable by software and not a hardware issue.

Give this is so easy to re-create a great shame support have you wastign time on things with it rather than re-create the problem and get engineering onto it.

New firmware today 3.9.1.3 - could be a fix but who knows! Cant test myself as ive had a refund.

 Improved: Make specific software adjustment for the newer version hardware GPY211
and GSW140.
 Improved: Improvement for the WAN up / down mechanism by ignoring the erroneous
port status update.
 Improved: Make sure the acceleration mechanism is aware of interface status (i.e. flush
related conntrack when an interface is down).
 Corrected: The issue of auto reset to factory default if the configuration was corrupted.
 Corrected: The issue that dial-in IPsec Aggressive VPN connection could not be up again
after a reboot.
 Corrected: The issue that SSL VPN LAN-LAN did not forward packets with MTU
1395~1400.
 Corrected: The issue of MSS (maximum segment size) in SSL VPN.

Buy another and give it a try

Not at the moment. If someone else confirms it fixes I may give it another go, but im not prepared to buy again and it not fixed.

Well thats interesting, i thought i might give this another go as im really not finding anything else that can do 10Gbe LAN from multi WANS, so I went to look at the normal places in the UK, but I cant find any stock anywhere even the legit UK store.
Wonder if there was a hardware problem and stock has been recalled, all very strange.

Mine is still ok, but what I can't abide is the constant reboots after any 'normal' change, as per any DrayOS based unit. For now I'll keep to the 3900.

Another problem I had was that auto negotiation speed detect on Port 3 (Ethernet WAN 2.5G) didn't correctly detect the interface speed correctly and it set it to 100Mbps (orange). I had to manually set it to 1Gbps under WAN >> General Setup - Physical Type.