I've now been living with the DS213Air for a few months and am pleased overall.
It turns out that the media server doesn't work when in Wireless Router mode - which I'd only changed to from the default Hotspot mode because of poor performance (like unconnectable between floors).
A bit of time spent with my Google Nexus and the free WiEye wi-fi scanner tool has revealed a massive overlap on a number of channels, it also shows just how much wifi on one channel spills into others. Having identified a pretty quiet channel the hotspot mode is now usable on the diskstation which means that media server is working perfectly now.
I've used the Download Station and have just started downloading RSS feeds to keep bandwidth out of chargeable hours which is a nice automation benefit.
I am also hosting all of my music and movies on the device which means less data stored on each PC in the house.
I have installed the DS Photo/ File/ Audio apps on the Google Nexus, they work fine locally but my ADSL router is to old to handle automated configuration for remote access (pretty sure it could be done manually if necessary), besides which DS Audio which is my main interest is too simplistic (missing Album Artist view for starters) to be of much use so I have been using Bubble upnp to connect to the Mediaserver product and that covers photos, audio and video, matched with MX Player on the Nexus and most of my films play pretty well.
I have Cloud Station installed on my PC and it works fine but is limited to 3 shares each (so no good for synchronising my home folder, music, videos and photos, 4 things), it also requires that the Disk station share is configured first which means copying everything to the Synology first and then synchronising it back to an empty folder on the PC, which was bad enough with 146Gb of music, 1.5 TB of movies is a non starter, it also plays havoc with genie backup as all of that data is a change so gets backed up a 2nd time. I'm now using it just for individual items I want to synchronise, eventually I'd like to do my entire Documents folder but a little more confidence is needed first, time will tell. The software is also clearly still in development as it has a basic error in that every time I login it tells me that 17 files have been updated, it's the same 17 everytime because they were the last to change.
Data replicator has been deleted as it's a back tool not a synchronisation tool so not much use for me at this point.