The lowest version we see is version 4. We are pushing our customers to the newest version. Because of the S&S they have from VMware.
Also Windows 2012 support is sometime the reason customer have to update to ESX5.x.
We did an upgrade from 3.5 to version 5.1 last month. So we don`t have any 3.5 versions running.
We now testing ESX5.5 in combination with Veeam7R2. Especially the support for Exchange 2013 (and now Sharepoint 2013
) is a great advantage. Every customers is really impressed what Veeam can do. We give al lot of demos so customers can see what Veeam can do in real world instead of whitepapers. Veeam sells its self that way