Online index rebuild is a feature available from SQL Server 2005 Enterprise Edition. While it does sound useful and attractive to use you should be careful considering it as part of your database maintenance plan. If you use it as a regular index rebuild operation then you could end up with serious locking and blocking and you cannot be sure and define the approx time of index rebuilds, it could be different every time.
In one of my customers the online index rebuild of the database took between 2 and 12 hours! So we decide to refine the task and go for offline index rebuilds for most of the indexes.
Why is that happening and what is behind the online index rebuilds.