For those of you who haven't been paying attention in VMware, with the advent of vSphere (over a year ago), the official recommendation is to no longer use ESX, but now to use ESXi.
Why would I do something silly like that?
I'm glad you asked :-)
ESXi is lighter - not so heavy on the host box
No security console - aka, less security risk and things to patch
Less configuration options - easier to setup a new host
What are the downsides?
Oh, those... Well, if you believe VMware, none! If you listen to third party vendors, then you will hear a long list of agents that they've had to scramble to figure out new methods of doing. Some (like Quest's vRanger Pro) are switching to a VA (Virtual Appliance) format. Others are just pulling from the remote API calls. And others (like HP Systems Insight Manager or Dell OpenManage) are switching their management tools to use SNMP and CIM based calls for managing the hardware.
Why can't I just keep using ESX?
Oh, that's because ESX is going to disappear in "future major releases," so why wait until you are forced into the situation. Try to be a little forward looking; after all, isn't that why you're Virtualized in the first place? :-)
If you want to read more from VMware, you can go here:
VMware made it official that as of vSphere 5 (coming out July-August 2011), ESX will no longer be included. ESXi will be the only way to go. Better start migrating!
Read more about it here.