If you're someone who is fimiliar with ASP.NET, and C Sharp (C#) then this CMS is the CMS for you. Telerik has great components for you to develope with, and they've earned their reputation for creating quality products. Sitefinity is one of them.
The API is very extensive, and so you can do more than you think you could have. I've used DotNetNuke 4, and Sitefinity one for it's own features and extensive API and documentation.