I do have nimble in my environment which is not used it for SQL server but I can give some insight on how it works.
we're looking to separate out our SQL Server data from our other stuff.
As far as I know this is not possible. all disks are used for all access but data access always comes from cache (kinda like how SQL Server always reads from memory) so separating sql data from other data is not necessary as with most SANs.
as for the tuning, it has performed pretty good in my SQLIO test. I did not use the file that SQLIO creates as this is all zeros and compresses to nothing. instead I used a sharepoint db backup file to run a number of read tests( I chose a sharepoint db backup because they do not compress very well). I did eight different read tests and only the first one(8k random) was slow, the rest of the tests performed well. I have attached results.
Basically the nimble cache behaves like SQL Server buffer pool. If data is in cache, access is quick, if data is not in cache, access is slow but now that data is in cache and any subsequent access will be quick.