just in time
I just started upgrading my ESX 4.1 hosts (not ESXi) to ESXi 5.5 late last week..
ESXi 6 here I come maybe 2017. Well sooner perhaps depending if there is anything cool that interests me in 6. There has been no changes between 4.1 and 5.5 that I found compelling(by contrast there was a lot of good stuff between ESX 3.x and 4.0 for me), the only reason I am upgrading is because I didn't realize 4.1 went end of support last year. 4.1 has been so solid for me.. HP pesters me politely that it is no longer supported but they haven't yet outright refused to support me yet..
Looking at what this article says is in ESXi 6, I don't see anything that gets me excited.
Last week when I upgraded my first host in my main ESX 4.1 cluster I came across a bug where I cannot vmotion from a 5.5U2 system to a 4.1 system with a vmware distributed switch(I can go the other direction no problem), the VM loses all connectivity until it is moved back to the ESXi 5.5 host (powering the VM off and back on again doesn't help). Vmotion over standard switch is fine. So I'm expediting my upgrades so this particular bug doesn't impact me too much.
(loyal vmware customer since 1999)