I read/posted on Usenet a lot in the 90's (and even up until around 2005 or so). It was still pretty big those days, and usable despite the spam (which only started becoming a problem in late 90's, I think). There sure were a lot of strange newsgroups though. Some of them I went in and couldn't figure out wtf they were talking about. The technical newsgroups were very useful those days, and it's actually where all the initial Linux development started. And there was cool stuff like alt.sysadmin.recovery. :-)

As far as readers go, I've used Pine (logical first choice on shell account), tin, and slrn. Slrn was more efficient over a slow dialup connection, but frankly I prefer tin's interface the most. Actually I also used Netscape (v3) sometimes, as it had a built-in NNTP client. Very useful for those special "binary" newsgroups...

I really don't like the google groups interface at all. They should have just given direct NNTP access so you can use any client you wish (even if they embed text ads above or below the posts).

You can still read some open-source mailing lists via NNTP, as described here:
http://gmane.org/faq.php
You don't even need a Usenet provider for that. In fact, it would be entirely possible for someone to create a web forum, text board, or whatever that also provides NNTP access to the boards. I'm surprised nobody has done this yet (if they have, I never heard about it). It would be even cooler if they also provided telnet BBS access to the same boards. Generally you only get one of those three, and most often it's the first...

I've checked out Usenet recently (I switched providers to one that has a news server) but all my old hangouts are basically dead. Oddly enough alt.slackware still has some activity, as do some of the roguelike groups.