You should go with what you are comfortable with.
You need to make backups whether the underlying storage solution is Linux based, Mac based, PC based or appliance based (for what it's worth Drobo also runs a custom linux distribution). Synology does not "need" to be backed up because it's Linux. It needs to be backed up because if you have a fire, theft, flood, or massive system failure you could lose your data. The same is true of WHS, Drobo and all of the others.
It is true that if you build a WHS based solution and the system fails, that (assuming you have them in good working order) you can take the individual storage drives and recover some data from them. However, this still does not protect you from things like data corruption (WHS doesn't even detect corruption or "bit rot" which the better solutions all do), fire, theft, loss, etc. It's also worth pointing out that using SHR or RAID 5 you can take a DS1511+ with 5X3TB drives and get 11TB of usable space. With a WHS solution using redundant drives, to get 11TB of usable space would require 8 disks (4 disks X 3TB minus formatting). So, you will spend more on drives.
With any "linux" based solution, if the system fails and your drives are intact it should be possible to put those drives into an enclosure and attempt data recovery. However, if you are backing your stuff up (which you should be doing) this is less of a concern. You can spend money on a backup solution or you can take a gamble on your solution failing you, and possibly spending far more money with a data recovery service. This goes for WHS just as well as it goes for Mac, Drobo, Synology, etc.
Synology units cost more because they provide small/medium business level support, a 3 year warranty, the units are built with very good parts and they are very low power consumption (my DS1511+ with 5 drives uses about 40 watts and provides about 7TB of storage. My WHS with an external eSATA enclosure and slightly less storage was consuming over 100 watts). They are also very easy to set up.
They are not for everybody, but certainly, compared to a Drobo hooked up to a Mac Mini, the Synology would be easier to set up, use less power, be more expandable and I would imagine cost quite a lot less (Drobo + Mac Mini is probably around $1300 or so without hard drives.. DS1512+ is about $800).
Edited by jmpage2 - 7/9/12 at 7:15pm