I use Thunderbird for newsgroup access and as a calendar. It isn't great for either and support seems to really suck.
There were some problems with the calendar program that nearly made it unusable. When I dug in and learned how the support mechanism works I found this was a known bug that had been around for over a year. Multiple people had reported it with a few different facets, but basically the same bug. Eventually someone found a work around that involved doing something with some java that had to be installed in a very odd way. Fortunately my girlfriend is a web developer and was able to help me out with it and get the durn thing to work again. So I was happy for a while.
More recently I am finding that the newsgroup support for offline use is amazingly buggy. Some posts never get downloaded unless I go through them one at a time while online. No matter how many times I tell the program to download the group, those messages will not be available when I am offline.
I guess that shouldn't surprise me. The first four items in the Help menu require internet access to work.
Grrrr...