I recently updated my VMware certification from 5.5 to 6. My 5.5 was expiring so it made sense to do the delta exam and upgrade, rather than recertify the same level. I realise I've done this just as 6.5 is coming out, but I've been using 6 lately so it made sense to me.
A lot of the maximums in VMware have been increased, and a good summary of that is available here: http://www.virten.net/vmware/vsphere-5-5-vs-vsphere-6-0/
One of the areas of most interest to me was the big improvements in VMware Fault Tolerance (FT). A couple of years ago I was investigating multiple options for a high availability (HA) VPN solution, and looked into using the CSR1000v to terminate the VPN's. The idea was to have one Cisco device and let VMware FT handle the resilience. The advantage of this would have been only purchasing and licensing one CSR, and not having to worry about any kind of stateful IPSEC synchronising between two devices. One of the main issues we had with this was that to get decent performance out of the CSR we needed multiple CPU's, and FT wouldn't support it. In version 6 it is now capable of up to 4 vCPU's. This improvement has potentially made the solution worth exploring again if I ever needed to.
Obviously there are a whole host of other differences, but there are hundreds of other sites out there to review them all on so use Google!
Thanks to Keith Barker (https://twitter.com/KeithBarkerCCIE) at CBT Nuggets (www.cbtnuggets.com) for the useful videos!