Enterprise authentication server

OK, here is the biscuit. Its fairly ordinary - you've probably got one
at home.

The Scenario
------------
1 Network. Its got a bunch of things on it. Some are PCs with
Microsoft Windows, some have Linux, some have Solaris and there is a
splattering of Mac OS 9 & Mac OS X. Total size ~ 100 systems.


The Thought
-----------
I've been looking into implementing an enterprise authentication
server. Ideally everyone, whatever OS they are running from the list
above, should use the authentication server.

After a bit of searching, it seems there are a number of options:

NIS / NIS+

NIS seems great. Just what I want. Insecure, but then this is going to
be behind a firewall anyway and any users are easily within range of
my stick if they are bad.


LDAP

Seems OK, but a level of complexity higher than NIS.


KERBEROS

Appears to be another level of complexity higher still, but seems to
be the most interoperable method.


ACTIVE DIRECTORY

Use Microsoft to control everything? Hm. Only if it suits the criteria
best.


The Question
------------

Does anyone run an enterprise authentication server in a similar
network? If so, then :

What do you guys use?
What have been your real-world experiences? Was it worth it in the
end?