My company does Google Apps migrations as a service to ISPs and other companies. Yes, it is sometimes tricky. However, it looks like Sky did the right thing here in providing a service window for people to be able to access their old email, and use the new Google Apps (Gmail Client). There are some shortcomings to the Google Apps API that make some bulk management tasks cumbersome, but they are improving it all the time and for the most part the advantages to the ISP (low cost, reliable service) and the user (user experience and storage) are well worth it.
I blogged about this today over here: