We support all versions of ESX. I just heard from someone that HP solutions are not using VMware API to get the performance data and instead rely on console agents and SNMP. Which means limited/no ESXi support, and lack of performance data shown.
As for built-in ESX VC, have you actually tried to use it for monitoring? Basically, it is just not a monitoring application, I was having hard time using it on my test lab hosts with just a few VMs because of no consolidated views, no summary dashboard, no ability to add standalone hosts into the picture etc. There's too much stuff to list here, I guess it is hard to explain until you actually try to monitor/troubleshoot your environment using VC.
As for other major features - if you'd like, you can review Veeam Monitor datasheet. As you will note, most of them are simply not provided by VC. http://www.veeam.com/vmware-esx-server/ ... tor_vi.pdf
I suggest that you give Veeam Monitor a try yourself - unlike some other monitoring frameworks it takes minutes to install and get going. Or, you can request a live demo with our Systems Engineer here: http://www.veeam.com/register_for_a_live_demo.asp
Hope this helps!