WordPress MU or multiple installs of WordPress?

It sounds like MU might be worth the effort for you. It’s main limits are that individual blogs have to use a theme which you have installed and turned on (i.e. they can’t add their own or modify them aside from the widgets) and that the same applies to plugins.

In favor of MU is that if you upgrade the core system it will be just one set of files to upgrade, rather than doing the same thing five times. Also, if you’re using the same theme for all the blogs then you can control it from just one location, so you can add features/plugins/whatever really easily and quickly.

The tradeoff in your case is mostly in terms of less documentation being available for MU, but even there, its a lot like single-user wordpress in most regards, and there’s a forum for MU that has lots of answers in it.

Is it possible to have multiple installs of wordpress that not only are on the same database, sharing the same users but also share the same admin?

If you use WordPressMU then there will only be one admin (the admin panels are seperatet but you can acces them all by logging in once and you can find the ‘backend’ of all the blogs through the admin of the main one).

If you mean individual wordpres sites then there is no easy way to share one admin account across several wordpress installs, unless there’s a plugin I don’t know about (though pan-install stuff is really complicated, so I doubt it)

Is it possible to enable themes on a per-user basis?

I’m looking for a solution to ease the upgrade process of the 27 WordPress installations I manage, but each would need its own theme and to not have access to the others, is this possible?

I believe that MU does allow each individual blog to have it’s own theme. You can specify which themes will be available to all the blogs, and then each blog can pick a theme.

MU is not necessarily a better choice than multiple installs of standard WP.

If you are looking for a way to control multiple blogs that will be setup almost identically, and/or you want people to be able to create their own blogs on your website, then yes MU is probably a good choice.

If you are managing multiple blogs that have different requirements for setup and functionality, then keeping seperate WP installs may be the better choice.

Here are some of problems I encountered while spending a week playing with WPMU 1.3rc3:

(1) When you do the initial install, you create a main blog in the install directory, and all other blogs added are added as a sub-directory (if you use the subdirectory method) under this directory. This was a minor problem for me, because I could not install WPMU in the root of the domain, so I put it in mydomain.com/info/, which means all the other blogs are a sub-blog of the main one (ex: mydomain.com/info/news, mydomain.com/info/events, etc). This may or may not be an issue for some people.

(2) User registration for all the sub-blogs doesn’t work the way I need it to work. When you go to a sub-blog and want to register as a user for that particular blog, it actually forces you to register on the main blog, and then the user has to be manually added to the sub-blog.

(3) If you want to disable user registration on the main blog, then it is disabled for ALL sub-blogs. You can enable user registration on the main blog, and then enable/disable as you like on sub-blogs. This is obviously tied to issue 2 above, but not an acceptable option for everybody.