This is the firs time I've tried ESXi and I'm still playing around with it learning how to use it. So far, I haven't been able to use it very reliably. I installed it then installed a vm with Ubuntu and a vm with Windows Server 2008. Eventually it started giving me an error "server took too long to respond" when trying to log into vsphere. The built-in webserver on the host didn't work either while that was happening. I reset the settings on the host machine and lost my vms and datastore. Trying to re-add it didn't work, it would just hang at the third screen on the add storage wizard (saying loading without ever doing anything). I had to reinstall ESXi and both vms to get back to where I was. I then installed some programs on linux to test them out. At the end of the day (to test it) I powered off the machine while it was in the middle of being used using the power button. Now I come back and turn it on and both vms are inaccessible (that's only when it doesn't give me the server took too long to respond error) and I can't add a datastore (which should already be there). Is this program really that unreliable? What am I missing here? I can't imagine actually using it in production if it's going to have my vms disappear and not respond like this.
First I want to confirm that you aren't seriously overallocating resources to your Guests and overwhelming your Host. Start with 1 vCPU and a small amount of RAM for each Guest.
If the above isn't the problem, then it sounds like a hardware issue on your Host. Perhaps disk or array controller problems?
You could log into the console and look at the /var/log/messages file to see if there are any errors logged. (Instructions for accessing the console are here: http://kb.vmware.com/kb/1003677 )
I don't know if the vms are using too much of the available resources. I get an unhandled exception in vsphere trying to view the resource allocation tab of the inaccessible guests, and I can't change their settings right now anyway. Also, it has an event that says "Configuration file cannot be found", I'm doing a config reset now. I believe I had 2GB of ram allocated to each guest (4GB total on this machine) so that was probably too much. I don't know what the cpu settings were, but it's a core 2 duo machine. I'll try logging into the console now.
Edit: Ok, I have the message log up. I don't see anything from yesterday, which is odd. I don't really know what to look for on here.