I have built a number of sites using Kentico, and overall the experience has been positive.
Their support is really great, they respond very quickly and will not give up until you have an answer. If you don't have a maintenance contract you can ask questions on the forums, and they will always try to answer your question and help you with your issue.
For end users the UI is a bit complex, at any moment they might be looking at several levels of nested tabs. So be prepared to train your customers (for which you can rely on official training partners if you want to).
The community is not so strong imho. The amount of extensions on the marketplace is limted (mostly templates), and I haven't found that much blogs about Kentico. The best thing you can do is follow the official Kentico blogs, since they will point you to other blogs as well, like these blogs.
One of the reasons the third-party development is lacking right now, is that the API is not that well designed, and you'd constantly need to recompile and release extensions after each hotfix (weekly). An example of this is the excellent Marker Kentico Business Library, which was released some months ago, and hasn't since been updated to work with the latest version of Kentico.
They are planning API improvements for v6 so hopefully that will improve the situation. Kentico CMS really needs to get a better database abstraction layer, preferably using LINQ.