I won't repeat much of what's already been said in the other reviews, but to summarise, I've been using the ReadyNAS for over a year now, so my review is based on proper hectic usage rather than out-of-the-box or 1 month usage.
Overall, i'm *very* impressed with its performance and ease of use. It's solid, sturdy (bit heavy but i like).. It's cheaper now (almost half price) compared to when I bought it, but I still find buying the bare-bone (RND4000) and HD's separately much cheaper. If cost still puts you off, I can assure from personal experience that it's definitely worth every penny. Back then I bought 4x1TB disks but for the same price you can get 4x2TB disks. There's a list of supported disks on their website, but I found Western Digital disks are very reliable and work like a charm. You may get occasional popups about error counts increasing, but it's perfectly normal, unless you're getting hundreds or thousands a day...
Raid X is truly amazing, fully redundant.
The main reason I like it and chose it is that it has proper support for Mac OSX, as well as Time Machine. I allocated 600GB on it for Time Machine backups, and backup my 2 MBPs through my Gigabit Ethernet with ease. In addition, you can setup shares with AFP (Apple's File Protocol) and NFS (for Mac and Linux). I found AFP to work beyond my expectations in terms of speed and reliability. I also enabled SMB file share for such apps like XBMC and visitors with Windows OS.
And for you tekky people out there, I moved my VMware images onto the box and run them from my Mac with ease, and minimal lag/latency.
The hard drives are quite cool, even though I have it constantly on, and constantly reading/writing to it. And the ReadyNAS widget for OSX along with automated emails keep me updated about temperature, fan speeds, array status, and any issues that may arise.
Its interface is a + in my eyes. Very easy to use, update, enable addons, etc. And for those brave enough, SSH is also useful and possible (it's running a linux Kernel).
The modules it has are also interesting. I finally can switch off my Macs and use the Torrent client within the ReadyNAS.
A year on, and the only disadvantages I saw that made me give this 4 stars instead of 5 are as follows.
* It comes with little RAM memory; I think 256MB RAM. Although it's good enough, but it'll take *forever* to sync arrays, repair, check health, etc.. I would highly recommend to purchase a 1GB stick of Kingston RAM, which is fairly cheap, and worth every penny. Replacing the RAM is very easy and there are tutorials on youtube on how to do it.
* Even though I've taken care to clean and de-dust it regularly, the fan inside starts to get noisy with time. Now, every 15 - 30 minutes, it emits a loud hoover-like noise for 2 - 3 minutes. Loud enough that I had to move it away from the living room. I haven't enquired to replacing the FAN, but I'm sure it's not too hard.
Overall, I can safely say that I'm happy enough to purchase a second one if I had to. Coming from a person who manages his own Linux server with 10+ disks, I would gladly sacrifice or make the server redundant later, while swapping it with a ReadyNAS or two.
* Much, much easier and ergonomic to manage the disks physically.
* Managing/repairing Raid arrays is a pain, and ReadyNAS just does it all for you. It hasn't let me down yet.
* No worry about updates, unlike my Linux server where every so-often an update breaks the system and I spend a day or 2 repairing it
* No worry about power cuts affecting my arrays, and spending a day or two figuring out which disk belongs to which array
* If it costs a bit more to make life easier when it comes to the above, I would gladly do it - and I did.
I hope this review helps anyone. Happy to comment more.