We have just been through this last month with a stack of users.
Of course start with the 1st rule of migration, backup, backup & backup, verify the backups, play with a copy of the backups & leave the original alone.
We spoke with all the users & asked for their help to clean out their exchange mailboxes well in advance of the migration, we set limits of around 50mb on the size of their *.ost's & locked the buggers out if they got over 75mb. That way they know your serious about the clean up.
We posted the size of everyones .ost daily until they cleaned them up. All users were very helpful & wanted to get below the 50mb mark.
Then we logged into each *.ost on exchange exported it to *.pst & placed the *.pst in their user directory on the e-smith server.
It took around 2-3 hours to complete including the client configuration. Exchange can be a bitch so remeber the 1st rule, backup, backup & backup, verify the backups, play with a copy of the backups & leave the original alone. If your not comfortable pay the money & get someone who is, it is cheep compared with the cost of lost data.
They have been running this way for over a month without any issues, other than they miss there public folders especially
shared contacts list
shared calenders and
task delegation
We are still looking to find the right non pay groupware alternative to run with e-smith.