I tried a lower model synology 4xxj and returned it for the readynas ultra plus. The thing is built like a tank, had a more powerful CPU, and native squeezebox support. Maybe the 1512 is just as powerful as the 6 ultra plus, at the same price. Is it?
I did like the synology UI. Very nice.
I guess my point is to make sure there are not any additional features you want before the purchase. I wanted native squeezebox support (it was available for the synology at the time, but wasn't updated as frequently). Plus, the read speeds weren't as good as the ultra plus. Granted, that was the lower "j" series, but I will say that I love the ultra 4 plus an the performance is phenomenal. UI for the synology is certainly slicker, but I don't use the UI much (set it and forget it).