Here is a great blog post about the transition from WP to EE written by one of our designers - http://www.viget.com/inspire/wordpress-to-expressionengine/
and here is another unique EE site that shows the flexibility of its native theming system -
www.teamviget.com
Template processing is slow, processing order is wonky, and it's very easy to create N+1 query loops that can only be avoided by writing custom SQL queries/php. They also made some poor database structure choices in certain areas.
Mostly though it just boggles the mind why they put so much effort into creating a regex-based templating system when the app is already just PHP.
Still, probably the best CMS I've used and it certainly has its uses. I just wouldn't use it to develop anything complicated ever again.
Might want to revise that to "from WP to EE."
In addition to repeatable fields, they're planning a versioning system, staging states, and even a forms builder. Not exactly earth-shaking for a normal CMS, but these will open up all sorts of possibilities for ProcessWire.
At its core, ProcessWire just gives you an extensible, hierarchical (and relational when you need it) model to build upon, and then gives you a really slick jQuery-style syntax by which to access it. You build templates in PHP, but the selector syntax is so easy to work with, it brings to mind WordPress theming.
One of its founding principles is that it is markup-agnostic. I'm using it right now for an XML source that feeds a site (and possibly later a mobile site, an app, or anything), with the XML being a ProcessWire template.
For these people, I would strongly recommend a look at ProcessWire. It's kind of like a radically simplified Drupal that is at least as easy to use as WordPress, if not more so, and has a similar attention to design and detail. I've been using it for a few months now across two very different projects, and I'm wishing I'd found it earlier.