For some reason IBM/Lenovo stopped updating that page (amidst several other important pages since the transition). You can find newer ESXi customizations under the individual support/downloads pages for the servers. As of 5.5 U2, the customized bits are rebranded as 'Lenovo'. Still nothing for 6.0 yet. As mentioned above, they are generally several months behind on releases, including update releases. This usually places them last after all the other big vendors.
Regarding running stock versus customized.. I've been burned by running stock in the past, but things may have changed over the years. Just be sure you have it on paper that IBM/Lenovo will support you on a stock release. Otherwise be sure that your management is ok with the risk of non-support (again, on paper.. print out the email!).