I have a requirement to setup a website that allows users, user blogs, a forum and is flexible enough to add other features via .net. I'm just about to evaluate Umbraco, but for another website that's clearly up the CMS alley, however the aforementioned project needs faster turnaround.
Umbraco is great for programmers, though (IMO) not so much for people less technically inclined. It does cater for all the things you have described, though in my experience, the relative lack of documentation make it a bit more difficult to work with users/groups and permissions (this is users and groups of a website not the actual Umbraco app).
Otherwise I found Umbraco to be great for any type of site and it is my CMS of choice.
NOTE: The last time I used Umbraco was about a year ago, so there might be much more docs now days.
Umbraco supports users, as in backend users with various editing and publishing permissions. There are a couple of blog and comments packages for backend users. Umbraco v4 also has Canvas (editing in place, within the website).
It also supports Membership which is front end website 'members'. You could provide blogs for these 'members' using an extension like Doc2Form.
Umbraco v4 now uses the standard .NET login controls so it's fairly easy to set up membership & registration.
For a forum Umbraco typically is paired with YAF. There is an article on how to do that at http://www.createsoft.co.uk/blog/
That article describes how to integrate YAF as a .NET control in Umbraco. If you are using Membership for other things, the forum will use a seperate username&password to the membership id. You'll need to ask on the Umbraco forum for info on how to get around that (it has been done)
It is easy to use or develop .Net controls in Umbraco. YAF and Doc2Form are 2 examples mentioned here.
For learning, umbraco.tv has been great. Its worth a subscription for a month or two at least just to get up to speed quickly.
Umbraco is a brilliant CMS - my personal favourite. It can be a bit over-whelming at the start (especially if you don't like XSLT) but it is so flexible and can do anything!
- Umbraco has built in membership system which is very easy to use and you can also use custom .net membership providers.
- Blog4Umbraco is a great Umbraco blog package (don't use Doc2Form!).
- Whilst YAF is good, there is a new forum package for Umnbraco called uForum - this actually powers the OurUmbraco Forum.
- Umbraco can be extended easily via .NET controls or XSLT (it has a full API).