We're running AD on a mixed 2000/2003 platform, and just recently introduced a Mac to our network.
I know that OS X 10.4 has built-in direct support for Active Directory, and in our testing, it has worked reasonably well. The Directory Setup tool is in /Applications/Utilities, and allows you to basically add the client to the domain in a very similar manner to adding a Windows 2000 or XP client (with a few extra options available). Once it's a domain member, you can log in with any domain account and the Mac treats it like a local account (with a local OSX-type profile). If the account has a Home Directory configured, it will place a shortcut to this at the bottom of the screen.
To access Windows file servers, you need to make sure that the servers are set up (through either Local or Group Policy) so that the following Local Security policy:
Microsoft network server: Digitally sign communications (always)
is disabled. Once you've done that, you can use OSX's Samba client (via command-K) to access the shares on those servers. By default, it should use the username and password with which you logged on, and will prompt you for a separate login if that account has not been granted access to the share.
Supposedly, Entourage (from the MS Office 2004 suite) gives decent access to Exchange (like Outlook), though I haven't yet tested this. Outlook Web Access works just fine in Safari or Firefox, though with fewer features than if it is used on IE.
Does this help?