I've noticed some of those things too, but not all (it might be just me, in which case I must have done something accidentally to get things working). If you have upgraded and not done a fresh install (and thus managed all connections with the new nm right from the start), and want to use the new network manager, then you should let it manage all your connections (if this is not ok, you may of course manage them any way you like); this basically means that you should clear any existing configuration and then let nm configure your connections, as this way your only concern is to hold the nm configuration in place (no need to edit config files that way, as nm-applet is graphical). If a connection does not work well, simply remove its configuration trough nm controls and re-create it. In short network manager is supposed to be an automatic, graphical tool easy to use without lots of manuals and things to remember, so you should forget any command line games if using it. You asked about these things at the Ubuntu irc channel, right?
To the latter things you mentioned, one of us has made something odd. I have Ubuntu 8.10 on several machines, some of which are multi-user systems, and I'm pretty sure I've configured nm myself with more than one user logged in, trough "switch user" and all that - and it has worked. Can't remember if I had to configure wireless networks (i.e. click on the name in the list) for every user separately, but the only "configuration" in that would be typing in the wireless network password, which should be done separately for every user anyway, because not everybody is necessarily entitled to using a specified network just because one user is (and it is possible to make the system remember the password on a per-user basis anyway, so no problem there).
- if you upgraded, clear any previous configuration, including old configuration files
- if you have trouble with a connection, delete and re-create it
- wired connections without password protection should work automatically; at least this is how it works for me
- passwords should in any case be typed in (at least once) by every separate user instead of just one
- if network manager is a pain, one can get rid of it and use something else instead, like wpa_supplicant
I recommend you visit the network manager settings, delete every saved configuration and re-try. If the problems persist, you should of course ask how to fix them, and be specific on what sort of connection it is and what the problem is. It's not a problem if user B needs to input password that user A already gave, it's just a level of security
EDIT: I created a "mobile internet" connection (a mobile modem) yesterday, but the automatic style did not work; dhcp did not get the right dns server addresses, because network manager handled a wireless connection at the same time (which was trough LAN to internet) and for some reason it always tried to use a dns server from the LAN, which wasn't available if the wireless lan was not on. Solution was to clear both connections (this was an upgrade; on a clear install it would have been so right from the start), then create them again, then set the dns servers manually for the mobile connection. And guess what - worked like a charm
so yes, network manager is easy, but "old" configuration seems to trip it pretty easily..