I know there is one other thread on MPM-ITK on these forums but the last post was over a year ago so I thought I would start a new one..
In my current VPS that I manually configure I use MPM-ITK and I have to say I have been very impressed with it and how easy it makes things.. Sites run as the configured user and group, I can be sure that one site cant access anothers files, its great.. Performance seems very good as well.. Its much easier than the common suEXEC/suPHP combination because you just switch Apache to it and add one line to the vhost config..
I have tested MPM-ITK with php-cgi and mod_php (not php-fcgid yet because I have found mod_php to be extremely fast for the Drupal sites I run).. I have also read somewhere that it plays very nicely with PHP-FPM which is included in PHP 5.3.3 and above..
It was disappointing to see that Virtualmin, which has so much flexibility, doesn't support this option out of the box..
With the new ways of working coming down the line Virtualmin is going to have to offer the options.. MPM-ITK and PHP-FPM are absolute MUSTs for integration..