I feel like this will only ever be settled by some monster blog series, but I'll try the short version.
About what it does: it creates another Network with the same codebase and user database.
Think additional Network Admin to log into, separate set of sites to manage, potentially a different network admin or admins, etc. It is NOT for everybody, and it is NOT for you if all you want is for an individual site to have a vanity domain name.
About the various plugins: They all do the same thing.
Once upon a time (before WP 3.0), "Sites" were called "Blogs", and WPMU kept track of the blogs in a
wp_blogs table. And there was another table -
wp_sites - that appeared to just keep basic info about your domain.
There was only ever one row in the
wp_sites table, and no interface for making others. But, if you created a second row with SQL and made a few other changes, you ended up with a second site (now called a
Network) on a different domain.
So, in 2007 I released the
WPMU Multi-Site Manager to automate those changes.
Networks for WordPress is a descendant of that original plugin, as is
WP Multi Network. I can't speak for
Network+, but the description makes it look similar enough. The point is that any plugin that creates Networks has to make these same changes, because it's how the
Network feature operates.
About the plugin warning: BuddyPress.
BuddyPress creates global tables, so it isn't attached to a particular blog/site. So, if you activate BuddyPress on two networks, it can't tell which set of data it's supposed to be using.
Finally - thank you for the kind thoughts! TBH, if we didn't use it extensively at work (JerseyConnect is an ISP that also hosts blogs), I wouldn't have kept it up. You never know who will find the next bug. :)
I hope that shed some light on things. Let me know if you have any more questions!