I have done 3 SQL Server migrations from obsolete hardware to new over the past 6 months and have used sp_help_revlogin with little trouble to move the users and permissions over. (knock on wood). The only problem we had was some strange permissions stuff burried in msdb for some sharepoint logins, that I'm struggling to remember the details for.

The targets were always brand new clean installations.

What sort of trouble did you run into previously? (I'll search my notes and see what the details of the SharePoint service migration piece was)