I'm (we are) using the paid version at work (three domains) and the free version at home (two domains). I have one simple way to describe it: Love it! That's not to say that there is no space for improvement, there is; but it has solved many of our problems.
Google Apps solved a big issue we had with email sync on mobile devices since we never had Exchange. We were frustrated with Outlook (as a standalone product) and having to backup all those PST files. Google Drive has been very good for collaboration, because in most cases it eliminates the need of connecting to the VPN to get files.
Calendar sync and sharing has been great; and we are experimenting a lot with Fusion Tables a wonderful tool that opens all kinds of possibilities.
BTW, for those out there that are comparing costs between MS Office and GA, I will say that Google Apps are more than just an office package. If you're going to be fare in comparison, you need to add to the MS Office calculation, the cost of the email server; Outlook doing POP3 with whatever webmail can't never compare to Gmail in GA, and it is obvious that there will be no calendar sharing. GA also offers a few other perks that MS Office doesn't include: fast email search (Outlook search sucks, period), Google sites (helpful specially for small business), Forms (for surveys or data collection) that you can send to anyone you like and use it right on the browser, Google chat, Google voice and many others.
Making a case between MS office against GA is not merely a case of comparing the cost of each; you need to compare features, specially those that are important to you or can solve problems.
Edited for grammatical errors.
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