|Problems? Is your data what you think it is?|
At $DayJob we evaluated Metadot for a major rewrite of our user-interaction site. It exceeded all of our requirements _except_ being easy to translate/have multilingual sites. While having a content management interface in english is not important to us, the elements you place in the pages are (or were, at the time of the evaluation) not available in spanish.
IMHO, it's not trivial to add i18n as it is. Other than that, performance can be on par with other alternatives based on dynamic pages, depending of course on how many gizmos you put in your pages.
We ended up basing our site in http://www.template-toolkit.org and writing a few Template::Plugin modules (which we will place in the public domain once the rewrite is complete and the code is tested in production).
<plug class="PerlPlug">Please stay away from PHP based CMSes!!! - They're evil and chances are that they will become incompatible with newer releases of PHP.</plug>
-lem, but some call me fokat