The main thing I like about the NAS's you can buy, is size. They typically will hold 3/4 drives, and you won't find a PC case on the market that will hold 3-4 drives, and be the size of any of the commercial NAS options.
That said, I love the versatility of building my own. Since home made NAS's will be based on Linux or BSD (usually).. the things you can do with them is pretty much unlimited. I think both have their advantages and disadvantages
Below is strictly my opinion, not meant to be the authority on this issue, it was just how I looked at things when I was trying too decide on whether to buy or build. I admit that I was (1) Probably a bit biased and (2) I have/had never owned or used a "store bought" NAS.
Hardware (not related to drives):
Store bought -- Pretty much limited to what is inside the NAS build, case specs, etc.
Custom build -- Pretty much limited by your imagination and budget. Do you want 2 drives or 10, 1gig of ram or 16gigs of ram (yes, I've seen this)
Winner -- Custom
Store bought -- Installing "custom" services to integrate with the rest of the NAS will probably be fairly difficult/impossible.
Custom build -- Again, limited only by your ability and willingness to learn to configure services (depending on the OS you use, a lot of them may be configurable by the webUI, others may have to be configured via command line)
Store bought -- Technical support, documentation, etc.. should all be fairly good. If you have a problem, there's likely a technical support phone number you can call, or possibly a forum on the manufacturers website.
Custom build -- Big hang up for the custom build here. If your "google-foo" is broken.. You'll have a helluva time. If the operating system you install is fairly well supported, finding answers can be pretty easy, but still very frustrating at times. Initial setup can also be quite a bit more difficult (again, depending on the OS you decide to use)
Winner: Store bought
This will probably be the biggest section, as most people will probably buy a NAS if building one is going to be considerably more expensive.
Store bought: I've saw some store bought NAS's that were very reasonable, others that were super expensive. Probably how good a shopper you are will come into play here.
Custom build -- Again, it's all about the hardware you choose and what you need.
My Build would require 4 storage drives
A 4 bay ReadyNAS -- $529 (I think it was a tad more when I built, but this is the current price)
Case: Antec Sonata Proto $49 (this was on sale)
Motherboard: MSI 760GM $60
CPU -- 2.8ghz Sempron $49
RAM -- 8gigs of Crucial $40
Power Supply -- 500w Antec $50 (to be fair, this had been sitting unopened in a closet for quite some time)
Misc -- Fans, cables, etc.. $50
Time -- Took me about 60-90min to assemble, install an operating system, and install and format the storage drives to a Linux filesystem. After that, it was simply a matter of setting up services.. which I don't count here, as you'd have to do that with a ReadyNAS as well.
Total: $300.. probably figure another $10-15 bucks on shipping, tax, etc.. for a few items... so I'll be generous and say $320.
Is my NAS faster than a ReadyNas? I honestly don't know.. but I'm really happy with it. Now if you're the type who calculates some "hourly rate" into your builds... I guess if you're paying yourself $200 an hour, you could find the ReadyNAS wins here.
Winner: Custom* -- I'll give this an asterisk, as you may well consider your time way more valuable than the amount I saved building my own.
After looking at the above, deciding to build my own was easy.
Edit: In the interest of fairness, I forgot to add the cost of some cables, fans, etc.. to the price on my custom built NAS.
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Last edited by IndyGunFreak; 11-17-2012 at 12:20..