@sysrpl: I don't want to get into any endless discussions here, especially since it doesn't seem to me that most of those who commented have an interest in actual serious responses. But my experience is that XAF, XPO and the countless DevExpress component products that make up the UI experience of an XAF based application deliver unparalleled developer productivity when used correctly. Note the "used correctly" part - DevExpress have, to my knowledge, never claimed that XAF enables people to create applications they couldn't create in other ways. Instead, the framework takes care of the vast majority of repetitive jobs for you, saving a very large percentage of developer time.
In the interest of disclosure, as well as to explain the experience I'm referring to above: I used to work for DevExpress a few years ago and I was involved with XAF directly for a while as well. However, in the past few years I've spent much of my time with DevExpress clients who use XAF and other products in real world applications, large and small. It is certainly true, just like it is for any framework, that its strengths are not the same across the whole wide spectrum of applications somebody might create today. But in a nutshell I can say that any functional or performance problem you encounter in a typical business application project can be solved, while using XAF to increase productivity.
There's much that can be said on the topic - ten days ago I completed a training class on XAF doing just that for a week, with an audience of 22. I can't possibly repeat most of the content here, but one final matter: if your application stops working with four users, there's something wrong. And you know what? If you zip up your complete project and send it to the DevExpress support guys, like Dennis suggested, they will analyse the problem for you and tell you what can be done about it. That's what a good 3rd party tool chain provider will do for you, and DevExpress is the best.