Only drawback I can see is that if you want WYSiWYG editing you have to use Internet Explorer at the moment (text based editing in other browsers - but Java based editor is in the pipeline).
Typo3 offers about 1000 extensions. The Extension System is really good and simple to use. There are alternative WYSIWYG editors (typo-slang: "RTE") - like htmlArea which is cross browser compatible. Take a look at the extension list: http://typo3.org/extensions/repository/list/ 
Another BIG advantage is the documentation. Typo3 is one of the best documented opensource projects I've ever seen - see it at: http://typo3.org/document.../document-library/Matrix/
I love this cms

btw - there are some videos to get started: http://typo3.org/documentation/videos/




FlickR provides an API in various languages including Delphi, .Net and so on... 