One of the sites we have is
FIRSTConnects.Us, which uses Drupal. We originally used a custom PHP backend for this, but we switched to Drupal after scalability became an issue. Drupal is
very powerful (the White House uses it!), but while it made complicated/abstract things easy, doing very simple things became harder. A lot of this came from the fact that we were trying to use drupal modules that weren't really meant to do what we wanted.
Since most CMS's seem to get functionality through modules/extensions, I would come up with a list of features you want and then compare the modules/extensions for each CMS. This will save you a lot of time later.
If you want some help finding Drupal modules, I'm sure our webmasters could make some suggestions!