I disagree with tnorthcutt somewhat. Yes, authorities are trying to put a lid on the wild illegal activities that are going on in there. So the volume of these dubious channels will go down while using controlled news-servers.
However, there is a vast number of perfectly legal channels which can be very interesting. Looking at the very Wikipedia article that was posted by tnorthcutt, it's evident, that the volume is still growing. Exponentially! At least it was still the case between 2006 and 2009. The graph is a bit imprecise for a better conclusion.
On the usability side, the whole nature of usenet being pretty much plain-text only makes it very attractive. There are no annoying blinking advertisements. Most usenet readers make reading the articles really comfortable. Much easier in fact than many online forums.
You are correct that google groups (from hereon abbreviated as "gg") has done "something" to usenet. With gg, everyone can now easily access the groups from anywhere. With a somewhat familiar interface. Even though I still prefer a dedicated news reader to the gg interface, it's undeniable advantage is that it works pretty well even through corporate firewalls.
Yet again, a reason why usenet is now accessible by an even wider audience.
However, groups created in the gg "universe" will not appear on the regular usenet servers (at least not as far as I know). So it's become a somewhat split "universe". But it could be worse.
As you noted, some ISPs decided to deny access to parts of the usenet hierarchy. How much this will cripple the system, I cannot say. It remains to be seen. But given the wide range of topics, ranging from Gardening over to photography, cooking and gaming, you can find a group on nearly any topic. It would be really a shame if those all disappeared. And given the number of users, the chance is huge that there will be people that take over the ISPs jobs. As it has been seen in some P2P cases. You shut down one access route, and not a month later 10 new systems, sites and whatnot emerge from the cloud.
I am not particularly worried that usenet will disappear. Yes, it may lose some momentum, but it's here to stay. Too many geeks feel at home in there. There are already usenet servers which operate independent from ISPs. Some free of charge, some not.
If you're only in for the discussions, than go ahead and use Google Groups. If only to get to know the system. If you have set up you lair in a particular group, feel comfortable there, and have access to an NNTP server you might want to look into a more sophisticated reader. There's many good free ones out there.