I've been playing with WordPress for a little over a month now. I've learned a lot and I owe most of that to this forum as well as the intuitive design of WordPress.
That said, I think I'm going to be moving on to Mambo (or another heavier-duty CMS). Why? Because I was never going to use WordPress as a blog. I use Livejournal for my blog needs (have been since 2000) and was looking to use WordPress as a CMS for a graphic-intensive video download site.
I found myself, more and more, disabling built-in WordPress features (RSS feeds, calender, comments) and spending a lot of time looking for hacks and plugins to do all of the things I wanted. My index.php is barely recognizable.
Looking down the road, I believe it would be less than efficient to build everything in WordPress. Nobody wants me in these forums begging for a shopping-cart plugin! Yes the line between Blog and CMS is blurry, but there ARE some differences. I guess, in the end, I look at it like this: if your site looks more like a portal than a diary, than using blog software is probably not in your best interest.
I've seen portal sites built with WordPress, but they still lack so much of the functionality I will want to have. I plan on trolling the WordPress forums still, and if I ever decide port my livejournal to an independent blog, I know exactly what I'll be using.