Don't event get me started on the SEO benefits of WordPress... I know for experience that Google loves WordPress, and have had some excellent indexing performance.
I sound like i'm a salesman here - can I just clarify I am an independent freelance graphic designer and web consultant with NOTHING to do with WordPress core team!!
I personally have got on very well with it and am constantly surprised how easy it is to do stuff in! The development path has been excellent since I've been using it too, with some solid improvements rolled out.