We often get asked which content management solution is the best. That is a hard question to answer because it depends on what you are going to do with the site. If you are doing a simple blog, WordPress is your best and simplest option. The best way that I know of to explain why you would want a different solution, say Joomla! or Drupal is an analogy. If you are wanting to build a one time deck at your house and need something to haul supplies, a Ford F-150 will work just fine. If you need to build decks and more for people all over the region, maybe all over the country you really need a Freightliner, Kenworth or Peterbuilt. If this is the case, you should look at Joomla! or Drupal because it will be cheaper for you in the long run!!
We are experienced with many clients including the Federal Government that use Drupal and Joomla! Go check out www.whitehouse.gov and do view source. You will notice Drupal tags in the code. We did not personally do this site, but it shows how a high availability, scale-able, mission critical site in the real world uses open source content management. Many fortune 500 companies are now using open source content management also!