The wide-zoom is an obvious choice for landscapes and event photography. I use this lens extensively for interior, travel and wedding photography also.
On the 5D and 5D II the EF 16-35mm 2.8L really shines. I find the range very versatile and use the lens at it's full range. You can create some dramatic images when used at 16mm. In the image above I used the 5D II in an ice cave near Jasper and in the image below the old Dairy Queen in Nelson BC was shot with a 5D.
Many buyers will wonder if they really need the extra f/stop. I must admit that I often shoot this lens in the f/8 - f/11 range but for event, stage and wedding photography you often need all the speed you can get, such as in the photo below - a scene from a school play shot at 26mm, f/2.8.
Canon has released a newer model of this lens, the EF 16-35 2.8L II and I have often toyed with the idea of an upgrade. Some shooters will vouch that the lens was due for an update but so far I haven't found enough of a reason to upgrade.