There are mailbox migration tools by software providers such as Quest which you can use, but they cost money, so it really depends on how many users you have and how much time you have to do it in. My organaztion moved from exchange 2003 to Exchange 2007 in the space of about 6 months total. We were in no hurry so we simply used the move mailbox tools native to Exchange to do the moves. We had 10000+ mailboxes to move and we moved them by order of importance making sure all was well before moving more. I moved most of the mailboxes over in the last month, scheduling so many per night while I was at work and I sent everyone being moved an email notifying them of the move and the changes they could expect. I checked on them in the morning and never had any fail. What exchange version are you moving users from? That would make a difference on what you can use as well since you cannot move mailboxes from Exchange 5.5 to 2007 without a third party tool to migrate them.