For the most part... they're a waste of time. They've got good intentions, but they just don't have the features and structure needed to compete with any forum only script like XenForo, IPB, Burning Board, MyBB, phpBB, SMF, etc. There's a good post about it on a WordPress specific news site here:
Will any of them ever be viable competitors to normal forum scripts? Maybe if they had a lot more time and effort devoted to them, and a bigger, more determined staff team with some sort of original vision. But not one WordPress forum plugin is, as of this point in time, good enough to compete with any dedicated software or particularly useable for a large/active forum.
It depends on your needs. If you need a full-fledged forum with all the features, I would stay away from plugin versions of forums. bbPress can be pretty awesome, but mostly if you need a simple forum. Or if you know how to add features to it yourself — it's great in that sense because you can build a forum from the ground-up.
I'd probably just go with bridging logins with a good forum software instead. Forum plugins for WordPress are either too simple or require too much time to get where you want them to be.
In the past, if having a WordPress site and forum -- if they compliment each other. I've just setup some sort of Single Sign On (SSO) or OAuth Plugin so they can use the same login on both sites. So I've typically kept the forum and WordPress site separate, I rather have a full featured forum.
I am currently investigating ways to provide some interactivity to my WordPress web site (see signature). I have in the process tried-out the bbPress plug-in in a test-environment site, as well as alternative plugins that implement various forms of interactivity, be it question-and-answer plug-ins or social-media plugins, sometimes testing more than one together.
I seriously considered bbPress, but finally realized that it was not supported by the polylang plug-in that I use for my site to be multilingual: French, English, Spanish. In the meantime I realized that I did not want to maintain a threelingual interactive forum, in the sense that, I do not want to commit myself to translating the contents from one language to the other. Yet I wanted my users to choose the language of their forum interface. The forum contents themselves would be French. Or perhaps there would be a section of the forum for languages other than French too.
So I turned away from investigating WordPress Plug-ins, and began to experiment with the following full-fledged forum software: Vanilla, SMF, MyBB, and PhpBB. I installed each one of these using CPanel's Softaculus autoinstaller.
I liked all of these, but I dropped Vanilla when I realized that only the administrator could switch languages, but the users could not. So that left me with SMF, MyBB and PhpBB. All three allowed users to choose the language in which the forum would be displayed for them.
After Vanilla, SMF was the first I seriously considered to adopt. I found it quite interesting and easy to work with, but I received a few "no file found" message, in response to some some basic administrative tasks. As I did not want to reinstall it manually at this time, I decided to try out MyBB and phpBB. I'd come back to SMF later, depending on what I found with the others, I thought.
As I had read some negative reports about phpBB's complexity, I was expecting not to like it very much. In the end I realized that I was okay with the approach overall, even if it required me to manually download files and copy them in the right spot, or editting a line of code in a .css file, e.g. to change the logo.
By the time I got to trying-out MyBB, I had already become used to phpBB, and to like it. So I did not invest as much energy or time with MyBB as I have with Vanilla, SMF or PhpBB.
In all of this process I have observed that WordPress plug-ins such as bbPress do not and cannot be compared with the full fledged forums I have tried-out a little. But they could be quite useful anyhow, depending on one's requirements.
As far as the full-fledged forum software were concerned, Vanilla seemed a little ahead of the pack in terms of its ease of integration with mobile devices. But SMF, MyBB and PhpBB were quite close to one another with regards to what functionality they had to offer, although not identical.
A final thought on WordPress plug-ins: There are several options available to help making a site interactive, but one has to look beyond forums into social networking plugins or Q&A plugins for example. One can maximize a WordPress site with such plug-ins and also complete the site with a stand-alone forum software, depending on one's objectives.