I'm about to order a NAS and I'm interested in what experiences you have had.

My mind is set on the Synology DS1511+ or the DS411+II, the DS1511+ looks to be more value as one can easily expand to up to 15 drives.
The Synology "DSM" software also looks to be stunning and the V4 Beta raises the bar vs the others.

So, what else should I look at?
I basically need it as a backup "device" but the added functionality made available by the modern NASs for mobile devices and the possibility of using it as a personal "Cloud" has made me wanting as much as possible.

Links
DS411+II
DS1511+