Well, we're live now with two ts-ec879-rp - one with 600gb WD VelociRaptors and the other with 3tb Seagate Constellations - both raid 6. Each with a pair of 1Gb trunked (round robin) nics
Both are performing well - we would have liked to have had the 1TB WD's but it's a live site and we couldn't risk using unsupported disks.
We're running 15VM's on the 10K disks - DC's, exchange, oracle DB's and various other apps (helpdesk, print, analytical processing servers)
the 3TB seagates are just running a couple of test vms (app server + oracle DB) and a couple of file servers. It's also serving up a couple of cifs shares and a 5TB iscsi veeam repository.
All working fine.
Latency is low on the VM datastores and users have reported an improvement in performance over the old setup (although I suspect that this was because of rogue traffic shaping settings on the old esx 3.5 hosts) - either way, it's not worse...
One bugbear is that I have a rogue process on both qnaps (rpc.rquotad) which I have logged a support call for but have received nothing back from Qnap. God help us if one of these things goes down, support from Qnap seems to be non-existant.
I suppose you pay your money and you take your choice (we have NetApp on our larger sites - support is excellent but by god do you pay for it...)