It has been sometime, but I still want to tell my experiences with ESXi 4.1.
In the beginning, I was very annoyed from the fact that you really need an 64 bit System and at least 2 Gigs of RAM.
But the rest just turned out fine, I have to admit.
Theres an upgrade path available, so you can upgrade your ESXi 3.5 Machine via an Upgrade to 4.1 without losing your Machines or the need to reinstall.
Actually - it worked, which did surprise myself in a good manner.
In Terms of Speed and Stabilty I did not recognize anything bad.
Especially the new features were downright awesome:
I just love the USB Virtualisation.
I really hit the top by plugin in an USB Videocard to my ESXi 4.1, and adding this Virtual Device to an Vmware Appliance: Awesome speed, had realtime video within my VM.
( I used it to virtualize a bunch of Servers, Firewalls, Routers, Asterisk and a Webcamserver... was really nice running ).
The only thing I really couldn't test and missed was the PCI Virtualization.
That feature *should* work - but you need some kind of state-of-the-Art Intel CPU / Board with Virtual I/O... and I had an old Tyan AMD Dual Core Board, only... ^^'.
So maybe sometime later.
But after all, I'm really happy with that System, I could migrate an running Windows XP Machine to the ESXi4U1 and the Converter was really doing alright...