I took the opportunity to sit the WCF Beta exam a few days ago and I must say, it was very hard. Being beta, I don't find out the results until about 8 weeks later, but I reckon I didn't fare too well.
I did book the test a little impulsively, however I saw the opportunity to do it, and only had a free day to do the test that was a few days later than when I booked it, so my preparation time was very small.
Having said that, I still think it was a hard exam. There were a wide variety of questions there and the preparation guide listed here doesn't really do the broad range of topics justice as there were still items/topic in the exam that were not listed in the prep guide.
If I do the exam when its released, then I think I'll prepare for the exam for around 2 months before sitting it again. Note that I have worked with WCF a reasonable amount and have also taken Juval Lowy's WCF Masterclass course but I would still allocate around 2 months prep time on top of that.
One of the things that did irritate me about the test (and its not specific to this test either) is when you are tested on your knowledge of things like enum values. One of the questions had code samples that differed only in the value of an enum being specified. Now admittedly, if I had worked a lot with the aspect of WCF, I would probably know it, however questions like these assume you dont have the services of a compiler at hand. Even if I didn't know it, practically, it would take a around 5 seconds to see this error when compiling and fix it, and still achieve a quality WCF implementation. I just don't see those types of questions of having any real practical value.
Anyway, if your thinking of sitting the WCF beta exam, or waiting for its proper release. Allocate a decent amount of preparation time, as it is quite hard.