At Breakaway Republic we've been using Umbraco, the open source, .net based content management system for coming up to six years now.
We have Umbraco Certified Professionals on our team, with both level 1 and level 2 certifications.
It's a great system, with a very open and supportive community that is always ready and willing to help out with any tips or solutions for your sites.
This was made even more apparent after attending CodeGarden, the annual Umbraco conference held in sunny Copenhagen. It was great to meet Niels and Per, the founders of the system, as well as the many members of the core development team working tirelessly to make Umbraco even better.
We strongly recommend using Umbraco in your next project that requires any level of content management. It is extremely flexible, allows for complete control over rendered mark-up making it suitable for any design, and can be integrated and extended with any .net control, library or site.
And of course, we're not being paid to say this, so you know it's true.