I use a first gen Time Capsule with an external USB disk for TM backups. I also have other redundant backup methods like Carbon Copy Cloner and Crashplan.
I have two Synology DSxxxJ series (arm-based, low power) NAS drives. All my other NAS drives have bit the dust including an old Iomega, a LaCie, and two old Buffalos. I don't recommend bottom-feeding when it comes to NAS.
For instance, I tried a Seagate NAS only to find they wanted me to pay a $19 a year subscription just to use ftp. And that was ftp from one side of my house to the other side of my house. No cloud involved. I took that thing back so fast it would make your head spin.
One feature I require in a NAS is "auto power on" after power failure. This sounds like a no-brainer, doesn't it? It isn't. Another feature I require is BYOD (bring your own disk). After losing all my data when I mailed my first gen Time Capsule back to Apple for repair (power supply issue - the disk was still good), I realized that I never again want my data to fall into the hands of fedex or ups just to get a repair. A nice to have feature is DLNA because some internet-ready TVs offer the ability to stream movies, photos and music from either Windows or a DLNA compliant NAS drive. Synology drives don't come with DLNA installed and enabled by default but installing and enabling it only takes a few clicks.
Most of our older Macs have SSD drives in them, to squeeze a few more years' use out of the machines. I plan on putting NAS in our newer Macs as budget and time allows. I keep bulky files on external firewire drives. I don't bother with the whole optibay mod. I'd rather be able to pop in the occasional DVD so I can rip it, put its contents on my NAS and file the DVD in the basement next to the music CDs and vinyl records.
While Synology can be expensive, keep in mind you don't have to spend everything at once. Pick up a low-end unit like a DS112J (single bay) and put an inexpensive WD green drive in it. You can go back later and swap in a larger drive.
Another "(somewhat) poor man's NAS" option can be had by plugging a Mac formatted USB drive into an Airport Extreme Base Station.