Jacob is right about getting the cache size right. For initial sizing, the starting point for cache is advised to be 10% of the raw capacity of the box, though go for the most you can afford (it's easy to upgrade cache afterwards and they don't charge you huge amounts more than if you had bought that capacity in the first place, but you don't want to have to go back & ask for more capex).

Cache size is probably the most important part, after raw capacity (personally regardless of storage I prefer to base capacity requirements on raw not after storage efficiency figures) and IOPS requirements (different models have different max IOPS, which comes from them having different CPU power, tho you can upgrade controller at a layer date if reqd), then the network interface required.

Choosing a VAR that has installed them often is a good idea. If your VAR is on the ball, they will offer to give you a tool to monitor your environment to work out your IOPS / cache requirements, if not ask your Nimble SE.

Despite getting high cache hit rates, even when it falls through to spinning disk the read latencies are still lower than when my workloads were on a 24 x 10k SAS enterprise storage array from a different vendor, so I'm more than happy with it (write latencies don't use the flash cache, given the use NVRAM & are bound by CPU on the controller). That only works up to a point though, you want to have enough cache to avoid that.

Looking at VMware before and after latency graphs over a few months, the latency just flatlines once the DB workloads were moved over.

I'm not a SQL dba but used to be, the current dba is more than happy with the performance boost.

Like Jacob says, it's great to have such fast performance at a great price from a company I trust enough given they have been around so long now with so many customers.