The MS Exchange Team has a nice post (http://msexchangeteam.com/archive/2010/02/22/454051.aspx) on their site explaining the history of Single Instance Storage (SIS) and where it went!

Reading the article I couldn’t help think that many clients are still concerned about space on their SANs and fear the idea of placing anything on local disk.  The idea in the article is placing your production mailboxes on Cheap Disk.  The only time a client or anyone should consider this is where you are using Database Availability Groups and have at least three servers in the DAG.  This will allow you to go backupless but also have all bases covered in your environment surrounding performance and recoverability. 

My only other beef though is Archiving.  The biggest question I get from clients is, “can I have my production mailboxes on one database and put the archived mail on cheap disk?”  Currently in Exchange 2010 an archived mailbox is stored on the exact same database as the users’ production mailbox. I have heard rumors that this is being looked into for future releases but nothing concrete.   What this means is that using SAN space which is typically RAID 10 or RAID 5 requires expensive disk for the production mailbox, which by default (if leveraged) archiving is also placed on expensive disk.   Explaining to clients that they can use local disk, have HA, but need at least three mailbox servers isn’t easy.  Not to mention that if you want multiple CAS in your environment with WNLB you need separate machines.  This is due to WNLB and MS Clustering cannot run on the same server.

Otherwise, given the way Exchange is deployed these days it is an enterprise solution.  There are times where departments will be included in a single database and other times where users are stored based on their last names.  I do love what they have done with Exchange 2007 and then what they have also done with Exchange 2010.  I just felt that I had to put in my 2 cents on the SIS discussion since I felt the post was a bit misleading.  While the applications are true, we have to consider real life usage and still address client concerns.