At times as VMware engineers, you may run into situations that sort of take away the flexibilities that you have with your environment. For example, a request to attach a RAW LUN to a VM, will prevent you from taking snapshots depending on how the disk is configured in VMware. Though RAW LUNS may seem like a hurdle for some, it has its place in the virtual world. One of the advantages of RDM is that it enables your storage team to run their fancy management tools on the presented LUN. At the same time, you will see better performance in a high I/O VM with the intensive I/O being executed on the RDM versus a virtual disk. A database server with huge transactional read and writes may seem like a good candidate for RDMs. MSCS requirements will also lead one to look at RDMs.
I found a good tutorial on RDM at showmedo.com that I am sharing with you guys.