This tool has been around for a while, but still worth mentioning.
Before you migrate your mailboxen you can use this tool to your connection with Office 365.
This helps a lot in making a decent planning and make calculations on how long it takes for all of your mailboxes to get moved to Office 365.
The difference between a normal speed tests is that this tests the actuel up and download to and from Microsofts Datacenter.
The site is available for each Microsoft datacenter.
America : http://speedtest.microsoftonline.com/
The first page display a summary of your connection. QoS, Delay, Up and Download speed.
Second page shows the route with number of hops. There are also tabs available for a map, analysis and graph.
Fourth page shows the max delay for up and download speeds.
Fifth page shows a summary of your connection, if values are not OK it shows in an orange or red circle.
The sixth page shows advanced statistics for your connection.
With these results you can make a good calculation for migrating your mailboxes.