There is no support for serial and parallel ports on ESXi 4 host.
I was trying to connect a modem device which communicates via serial port on one of ESXi hosts at customer sites the other day. But every time when I wanted to add a serial port to my VM, which was an Windows XP VM with a Fax Software installed, the radio button was just grayed out.
The same VM could be successfully connected on another host (the same hardware model) which had been installed with ESX 4. Slightly disappointed with the fact that ESXi4 does not currently support Serial or Parallel ports. I was searching around and finally found an explication on why on this VMware KB. The ESX 4 is supported and worked fine for me. The KB was updated at February 11, 2010 which is quite recent.
VMware is actively working with over 250 partners to enable solutions that typically depend on serial port communication for ESXi. VMware ESXi is a more secure hypervisor architecture that enables third-party solutions through CIM providers.
This is another reason on why the mass adoption for ESXi 4 is just not there yet. Many of the SMBs an small shops does uses fax/modem devices. And sometimes not only one, but several on different physical servers. And when you want them to adopt VMware vSphere 4 with 2 servers and HA feature for example, you might take in consideration such a fact before you make your proposition.
Otherwise the ESXi 4 is great. The patching takes considerably less time, using VMA is quite OK since the commands you knew from the ESX 4 classic are the same in the VMA. You can also setup the FastPass feature on the VMA to be able to quick switch between your ESXi servers without the need to enter your credentials every time. Don't need any local storage either, because you can use the embedded version or the Hypervizor.
Source: VMware KB