My personal favourite for newsgroups is NewsMan Pro “Usenet newsreader and binary grabber for Windows”.
I use the paid version but there is also a freeware one if that is your thing. See the downloads page which contains a comparison of the different versions:
I tried a bunch of different clients before I found this one and they all sort of do the same thing but what really made this one stand out was the ability to use Microsoft SQL Server for message storage (ADS and MySQL are also options). Other than reading new messages one of the primary things I used the Clarion/Softvelocity newsgroups for was to access the archive of knowledge stored in there. Searching is awesome and fast as it makes use of Full Text Search features of the targeted backend.
The UI is reasonable if a little quirky at times. More than likely this is just because I have not bothered to spend too much time on configuring the layout, profiles and various other UI related settings.
I don’t really write posts very often but when I do it seems to work fine.
Downloading is fast, I have it setup to check for new messages every 30 minutes but there are various other schedules you can setup as well as a bunch of automation options:
Checkout the stats from the MSSQL Disk Usage report: