<p>I know you can use several different view engines with ASP.NET MVC:</p>

<ul>
<li>ASPX, obviously</li>
<li>NVelocity</li>
<li>Brail</li>
<li>NHaml</li>
<li>et al...</li>
</ul>

<p>The default ASPX view engine seems to make the most sense to me, coming from an ASP.NET WebForms background.  </p>

<p>But, I wanted to get an idea of the pros and cons of each and see what most people are using. </p>

<p>Which does StackOverflow use?</p>

Most people on the planet will just use ASPX because that's what they know.  Another excellent benefit is the compiled-nature... so you not only get type-safety and intellisense, but you can get the perf benefit as well.
The drawback that I see is that it's so flippin' verbose.  I converted an app to NVelocity and was astounded at how clean it looked.  The problem is that there were a lot of things that didn't work with NVelocity (like your own custom view helpers) and there was a severe lack of documentation.
I added a feature to MvcContrib where you can register your own HtmlExtension types to it, but it's more of a bandaid until a better solution comes out.