Chances are, if you're running an enterprise-class storage product, the "failure" of the product wouldn't be a failure at all. It would be an amber light on a controller, power supply, fan tray, or disk and a warning telling you that you should probably get it replaced. You call the vendor (assuming you bought support) and they ship you a new one. You slide the replacement component into the device and off you go. Production hasn't been impacted and you're back where you started.
On the low end, be prepared
If you're running a low-end device, you might not have a support number to call. Even if you do, don't plan on getting a replacement component the same day or even the same week. Worse still, don't imagine that the component you get is going to simply fix the problem all on its own. In many cases, a hardware failure in a low-end NAS/SAN device will necessitate a full restore of the data that was on the device onto a completely new device. Most of the devices in this space are not designed to be modular beyond having RAID and replaceable disks. If you have a controller failure, you're probably starting over with a backup.
The bottom line here is that if you have the time and availability to jump in and replace the device (perhaps with a spare that you bought because you were saving so much money), a low-end unit might work for you.
Gauging the cost of downtime
The other side of the equation is who is relying on the data the device is storing. If you're a larger business using the device to store your workstation deployment images, you can probably get away without having it around for a day or two while you get a new device and restore the data. If you're a relatively small business and are using the device to provide the storage for all of your servers (virtualized or not), you really need to look at how much the absence of that data will cost you.
For example, imagine a small business that needs shared storage for a virtualization implementation. Let's say the company in question is a local insurance agency with 60 employees. They underwrite with a bunch of different insurance providers and have a document imaging system, so they have more servers than the average small business, which makes virtualization attractive. However, the relatively low number of users won't put a big transactional load on the storage, so a lower-end storage device works from a performance standpoint. The question becomes, do they spend $2,500 (or even less) on a lower-end NAS/SAN solution, or upwards of $20,000 on an enterprise-class SAN solution?