I have done the first upgrade til vSphere 5.5 update 2d today, and that went smooth. Afterward I needed to show the customer some things in vCenter and i used the vSphere Web Client, afterwards his first comment was, why is the vSphere Web Client running so fast on my machine, I had not noticed it, so i tried i againt, and it seem faster.
To make sure, he also tried to use it on his one machine, and his conclusion was it's a lot faster then the old version.
The upgrader was from vSphere 5.5 Update 1.
I have not checked it with other installation, so it might just be that the upgrade cleared som cache og other thing, but my impressions is that it's running a lot faster.