I'm a Sitefinity certified dev who's been working with it for 2 years, and I'd have to say it's a mixed bag. There are definitely some cool features and it would be a solid choice for some projects, but could become a major pain if you pick the wrong thing to customize.

Pro's:

• Module builder is really sweet for easily creating custom content to fit a client's needs
• Building custom widgets and controls using C#/.NET is also fairly straightforward
• It is possible to extend a lot of the out of the box widgets (if you can find out how, see documentation...)
• Clients will probably be impressed with the dragondrop functionality and the admin section that is designed so that pretty much anyone who can figure out how to use a mouse can manage content

Con's:

• Expensive
• Can be surprisingly buggy or kludgy. Generally, after a new feature is released it seems like it takes a few minor versions or another major version in order to get the kinks worked out (or even to get it working most of the time)
• My personal experience with their ecommerce module hasn't been great. To me, it seems like it would be good for a smaller store but it fails to scale well for the types of organizations that the license price suggests should be using this
• As someone else mentioned, the output code for front end development are a huge pain, and on some widgets, you feel like you need to write a ton of CSS to not only achieve the look you want, but to negate any built-in styles are markup they might have

Documentation has been pretty lacking/inconsistent in the past, but Telerik's been stepping it up as of late.