I've done 4 MS Certs now and I tend to complete them in around 45 - 60 mins, there are between 40 and 50 multiple choice questions and you need to go to a Microsoft Certified training centre to complete them. They are not web based.
I largely feel MS Certs are worth it if not to go over material that you might not come across in your day to day job. But it also give you more credibility to a certain extent against someone who hasn't bothered to do any at all. In fact I see no reason why any serious MS developer shouldn't be constantly taking exams.
I'm a bit of a hypocrite because its been ages since my last one, but I have a SharePoint 2010 one coming up in a couple of weeks and I really haven't done enough revision.
I find 90% of the training material to be utterly boring to read and alot of the questions on the tests are obscure and stuff you would likely never want to know again.