any issues with VMware workstations on MS server 2008?

I am running VMware on an MS server 2008. I created several machines to run our accounting program, a file server, an access database. Each runs in a seprate Virtual machine. This has been working very well and had no issues.
Are there any issues with this setup? Should I move to ESXi server. ESXi has specific hardware requirment whcih calls for more expensive hardware. Also we have a special microsoft licencing $60 for a MS server and few dollars each additional CALC.