We had our first look at Hivemind Communities during Steemfest 4 and they certainly showed a new potential for content discovery and the possibility to find your place in the huge ecosystem which Steem has become.
Since then Communities were in a closed beta version, meaning that you could test them, but you needed an username and password to test them.
UPDATE: It seems access to beta has been restricted again. Probably too many people were messing around in the communities and things became less clean than they needed to be at this stage.
Since very recently, the closed beta became open beta, meaning anyone can go ahead and test the Communities. I have to admit, if there was an announcement letting us know closed beta became open beta, I missed it. But maybe they feared a huge influx of people trying out the beta without the infrastructure being ready to handle it at this point.
Anyway, if you want to take a look (even to create your own community), here's the link:
https://beta.steemit.com
Careful! For those who tested closed beta or read about the testing of closed beta, at this stage, if you post in a community from the open beta, the post will be shown on your blog feed as well on main interfaces. You cannot see it from the beta interface on your feed, but you will see it from the main interface (and so will your readers), because the main interface doesn't understand communities yet.
In fact, what does posting to a community technically means, from what I understand? It means posting with the community tag (i.e. hive-xxxxxx). The Communities code knows to show that post to the right community and not to the personal blog feed, the interfaces which don't support Hivemind Communities (yet), will simply show the post on your feed. In both cases, the post is on the blockchain, only where it is displayed differs.
I publish this post in the "Communities Feedback" community, but knowing it will go to my feed as well (warned by the experience of others who tried it before I did), I made so my readers understand what I'm talking about.
My first impression is that Communities will be indeed a great improvement in content discovery compared to the options currently available to us.
And it's already fantastic at this point if you subscribe to only a few (like max. 4?) communities with well-balanced content between each other.
What do I mean by well-balanced content? It's easier to explain what unbalanced content is and you will understand then. If one community is hyper-active and in another community posts are seldom, then you'll likely miss the posts of the less active community in the feed of your communities. You'd have to go to that community to see what has been posted. That is an issue for now, without proper sorting options for the communities.
Regarding sorting, filtering and searching in the communities explorer, the increasing number of communities will pose a great problem, in this iteration of the Hivemind Communities. I've seen there is a Github issue by 
If someone wants to join more than a few communities (10?), then things start to look a little bit like the current condenser and every post that is shown. But... there could genuinely be an interest in more communities, in which case a better filtering at the level of "My communities" page is needed.
Let's say one is indeed subscribed to 10 communities, but at no single time he or she wants to see posts from more than a few communities. A possibility to filter the posts shown on the "My Communities" page by using a list of check boxes (+apply button) filter -- which includes each community the user is subscribed to -- can make the feed easier to customize.
Overall, the basis for further improvements has been laid. We can always find things to improve, and if the basis is solid, then they will come, probably in quite rapid iterations.
I think communities will make us look at Steem quite differently going forward.
Content discovery is probably the biggest problem Steem has. With a better content discovery, we will have better retention, better onboarding (and something to push out there), and likely we will have a more fair distribution of value (although that I believe will always be debatable).
The ones mentioned above combined should lead to a better image that can be well marketed and we will benefit from for SMTs. And finally that should help with the STEEM price.
Does this make sense to you?