Synology is busybox-based, with md/lvm tools etc., and for the most part behaves properly. The GUI is handy for remote access and management (with self-signed cert), and is pretty functional for all but the corneriest of corner cases.
This past week I needed to ssh in in order to e2fsck my storage prior to lvextending it. Kinda disappointed I had to do that, but the fact is that I could and did. Also, since the RAID is in software, in theory I could pull my 7 drive RAID out and stick it in another linux box and vg(im|ex)port it.
My 1812+ has adequate power for pushing ~100+MB/s with its dual-core Atom and 3GB (it took a spare laptop SO-DIMM), and runs at a pretty low wattage rate vs. a handbuild mid tower. It can't transcode, but I have WDTV Live boxes that support most codecs fairly well for that.