Google Groups used to be a great service when they first took over Deja News, but things have been gradually deteriorating ever since.
They used to be read-only; now, they allow anyone with a fake account to post. They are a huge source of Usenet spam. I have been kill-filing posts from Google Groups for about a year. It's not a pleasant thing to do, since there still are quite a few people who I like to read but only see quoted in replies now because they use Google Groups.
Whenever someone posts the same article 10 times in a row on a Usenet newsgroup, I always look at the mail header, and every single time it is from Google Groups. I don't know what is wrong, but there must be some user interface bug that causes people to resubmit their post. Maybe they don't realize that Usenet has a lag between posting and seeing your post. Maybe there's something wrong with the submit button. I don't know, but I've been seeing this phenomenon for a few years now.
Initially, Google Groups made a clear distinction between Usenet newsgroups and Google's own groups, but they've been blurring the two more and more over the years. The result is that a lot of people don't even know they're posting to Usenet, and are unaware of the different etiquette and social customs there (real names, top posting, etc.). Lately they've started including third-party forums as well, so now it's basically a free-for-all, and there is no way to just search Usenet anymore. You have a choice between "Google Groups" and "all groups", where the latter includes Google Groups, Usenet, and random bulletin boards on the web.
If Google isn't interested in supporting Usenet with a decent archive anymore (and I can understand their reluctance, since I'm sure there's little to no money in it) I wish they would pass the responsibility to someone else and get out of the way, because at this point I wish Deja News was still around - I'd have no need for Google Groups anymore.
And I know everyone loves to bash on Usenet, about how it's obsolete and dead, but I have to say that at least when it comes to programming language newsgroups, it's still hopping. I follow about a dozen programming newsgroups, and I can't keep up. About once a week I have to "mark all as read". As long as it still causes information overload, I don't think it can be fairly declared extinct.