Yes, the offline takes the system down, ftp's and unzips all the software to the inactive partition, licences, reboots and then swaps the partition and then loads the backup and finally reboots to working.
Took just over 1.5 hours last night from MCD 5.0 to latest MCD7.0. So far no problems reported; even the SIP's came back up cleanly... One or two odd reports in maintenance logs but I put that down to the way it reboots at various times.
How long did your online one take?