Think what you have to go through to try out a collaboration application you've found that you think might work for your office.. Mac or PC, doesn't matter.
You have to install it yourself, check that it works (while praying that it doesn't mess up your settings or interfere with other programs). You have to convince others to install it so you can try it together (while praying that it doesn't mess up their computers). Client software needs a server somewhere to exchange information.. If the application/plug-in doesn't get much use, you will ask everyone to uninstall it, and good luck trying another one - you'll have to go through the process all over again.
Now, compare that with what happens in the Google Apps world.. You just add the application you like to your domain.. it connects to all other Google apps services that it has business communicating with (authentication, email, calendar, docs, etc..) Voila! You and your office are ready to go.. No additional software to install, no databases to worry about.. And if you don't like it - no problem! Just uninstall it - it uninstalls (unlinks rather) for everyone.
One such example is Gqueues - it's a nice task manager that's well integrated into Google apps environment. It's connected to calendars, it supports task assignments, sharing over Google protocols, etc..
You can find many more apps in Google Apps marketplace - https://www.google.com/enterprise/marketplace/
So there you go - Google apps is not just about cheaper online office apps - it's about a much better platform and an eco-system!!