I’ve done quite a few projects using Community Server. If you’re okay with the out-of-the-box functionality, or you don’t mind sticking to the version you start with, I think you’ll be very happy.
The times I’ve run into headaches using CS is when the client wants functionality CS does not provide, but also insists on keeping the ability to upgrade to the latest version whenever Telligent releases an update. You can mostly support that by making all of your changes either in a separate project or by only modifying aspx/ascx files (no codebehinds). Some kind of merge is going to be required though no matter how well you plan it out.
Community Server itself has been very solid for me, but if all you need is a blogging engine then it may be overkill. Skinning it, for example, is quite a bit of work (despite their quite powerful Chameleon theme engine).
I’d probably look closer at one of the dedicated blog engines out there, like BlogEngine.NET, dasBlog or SubText, if that’s all you need. Go with Community Server if you think you’ll want more “community-focused” features like forums etc.
You can also take a look at Telligent Graffiti CMS.
It supports multiple blogs and authors.
Update: It’s now open source and available at http://graffiticms.codeplex.com/