IPB is a nice piece of software. They've done an exceptional job on piecing together a product solution in parts, allowing a person to pick and choose. The only problem is that it's also bloated and very resource intensive. You would be better off running Joomla 2.5 and XF than their CMS + Forum product, as an example. The cost would hurt if the site became large.
Their backend is pretty tidy as well. Like anything... you get used to it from using it.
My philosophy is that a blank software should be fast, responsive and stable. Nobody wants boring default, so you need room to add third party features. IPB struggles in this area unfortunately. The default product is resource intensive, but manageable I guess. When you start adding features, you want to be prepared for a dedicated server much earlier than say VB3.x or XF software, either one using Joomla 2.5 as a CMS. Wordpress is a huge resource hog now too, unfortunately.
This is what I like about XF. The raw product is so damn fast, it's a rare software package. You then have room to add lots of features without killing yourself at the server. I had it working on a single core with 400k posts and 5k visitors daily, with moderate features. I've only recently had to shift it to a dedicated, though I can now add more features because I have craploads of headroom at the server, with a higher post count and nearing 7k daily visitors, and the server is humming along with 600 online, no great deal... packed with features.
I suspect if I was still using VB4.x, my current server would be bordering unhealthy trying to dredge the software along, in conjunction with all the third party features, let alone VB5.x. People will be needing massive server upgrades if they upgrade their 4.x site from the way that software performed when I viewed the demo. Yuk!
