Folders don't work the way you're used to
Unlike most conventional mail clients, Gmail doesn't have hierarchical folders, but instead uses labels. You can work on all messages with a particular label and treat them as if they were in a folder. Labels don't have a hierarchy as such, but a message can have more than one label and thus appear in both label views.
To map labels into Outlook's hierarchical folders, GASMO turns folder locations into fake hierarchical labels: A message in the InfoWorld folder will have a label of
Inbox\InfoWorld. This makes it possible for a message to appear in more than one folder in Outlook. If you delete one of the messages, all the instances are deleted, which is not what you'd expect if you're Outlook-oriented.
The messiness of the hierarchical labels could make it difficult to get used to Gmail after a migration; you start getting a lot of backward slashes in label names to simulate folder hierarchy, rather than the clean names of Exchange folders. If you decide you want to use the Gmail interface instead of GASMO, you may find it worthwhile to restructure your labels to the friendlier, folder-style names it supports.
Using the Google interfaces is not an easy adjustment
At first I wanted to use the Gmail interface, figuring I'd make a clean break with Outlook, but using it wasn't so easy. I missed features in Outlook that are weak or absent in Google Apps, such as Outlook's powerful rules. And the fact that GASMO was working so well for my wife led me to give it a shot; I'm still with it. The Gmail interface can be tolerable for short periods of time when I need it, such as on my Droid smartphone, but it is a chore.
Google Calendar is less capable than Exchange
I had also heard of problems with using the Google Calendar interface when compared with Exchange's and Outlook's. No question about it, Google Calendar is no match for the Microsoft product, but it has been good enough for me so far.
Google Calendar in Google Apps Premier is strong in many areas. It supports resource calendars, such as for conference rooms, but GAMME doesn't migrate them from Exchange. Making multiple calendars and checking them is easy, but Google Calendar lacks many of the complex features for meeting setup, such as optional versus required meeting invites.
One of the biggest Exchange features not supported by Google Apps is Public Folders. As a replacement, you could create a Google Group (basically a Usenet-like newsgroup). They are archived and searchable, but not as well integrated as public folders, especially if you're using Outlook. As an alternative, you can also share any file on Google Docs.
Bear in mind that Google is creating APIs for all its features, so you can write your own apps. And you may be able to find third-party modules for a missing feature in the Google Apps Marketplace.
Google Apps can work differently as time passes
Unlike typical Exchange hosting, Google Apps is a dynamic environment. Don't be too surprised if you wake up one morning and some feature operates differently. At one point during my brief test period, the online documentation changed in a way that affected my tests, but it's not clear if the product itself changed.
There are good aspects to this dynamism, but if you treasure stability and being able to test changes before implementing them, Google Apps may be a problem.