Hi I have used it, the company I work for had a website made and the developers used this so we could make changes, at the time myself and other colleagues didn't know much about making edits but this made it very easy for us.
A couple of friends of mine have switched from drupal, joomla. Thats why I'm wondering is it worth learning, given that I also use ASP.NET, or are there any catches? It seems to be backed by microsoft http://www.microsoft.com/web/umbraco
It's not gonna give you any awesome reason to switch languages for the sake of it, cut as you're already friendly with the language I'd say it's entirely your preference; it's a solid cms but it's nothing groundbreaking is all
It really depends on what your looking for in a CMS system, if it's out the box working with minimal coding then it should be fine.
If your looking for a more programmer orientated cms that fits in with the MVC pattern then check out n2cms, we're just about to release a large site using it to drive some of the content. You make your templates in code as decorated classes, no nasty xslt/xml but you need to know what your doing.