What you choose for your primary storage may be the most important purchasing decision you ever make. That storage will form the bedrock -- or the shaky ground -- upon which the rest of your server and application environment will be built. Primary storage is a major capital investment and frequently has a much longer lifecycle than most infrastructure components, often remaining in production for five years or more.
It's hard to know what you're getting without rolling up your sleeves and putting your hands on it. I highly recommend doing just that. Fortunately, almost all of the major storage players offer evaluation units you can test firsthand. At the very least, get one of each of the top options you're considering.
Get familiar with the ins and outs of the management interface, learn how easy (or difficult) the device is to configure, and if practical, do a head-to-head performance test. The evaluation process often brings up questions you would never have thought to ask potential storage suitors without seeing the product and making it work on your own.
Many times, potential storage customers will visit an existing customer instead of doing a full evaluation, but this is only part of the story. Yes, you typically learn some valuable lessons, but you won't find out how the storage will work in your environment.
Storage product lines tend to distinguish themselves from each other in their higher-end features: snapshots, replication, and in some cases, de-duplication. Run these features through their paces and you can get a much better feel for the functionality and determine what you really need.