I'll chime-in with my set-up:
I have a Synology DS411J..... the cheap 4 bay one from 2011 (about 200 quid without drives I think?).
I use it to house my iTunes library, which then streams to laptop and apple TV.
I also use it as a destination for my time machine backups of the stuff on my laptop, and for a backup of the stuff on my girlfriends laptop too. I realize that this setup doesn't give me a complete backup of my iTunes library (just redundancy). But there is nothing on there that couldn't be relatively easily replaced - it's a calculated risk compared to having to backup several TB of content.
the drives don't need to match - you can put anything in there that you have lying about (I already had several externals that I was using as a half-arsed storage solution).
The speed seems fine - there is a slight lag compared to hosting my iTunes on a local drive, but not much. My friend bought a cheapo Buffalo, and it was virtually unusable. The synology streams 8gb 1080p video, no problem.
It always seems to be visible to all devices on the network.
It's easy to set-up and manage - Its been virtually untouched for 3 years.
I don't use the Synology apps, but the basically give access to your content via an app, rather than explorer. Which could be useful for "devices".
You effectively lose the capacity of your largest disk in the array (common to any raid set-up I believe)
Any disk that you put in it, gets formatted as part of the installation process. If you only have 1 or 2 disks at the outset (that are housing your data) you might need to buy/borrow another one.
I've had 1 drive crap-out in all the time I've had it. Of course it was one of the largest ones but the unit sent me an email to let me know, and I just stopped at a shop on the way home and bought another to replace it. Two minute job to replace the dead drive, and it just re-built itself overnight.
Anyway - that's probably way too much detail..... but in summary:
The synology, it's good.