For the most part they are a good option for many organizations.  The number of online auctions being added to existing events is growing everyday.  Several reasons:

• Additional revenue stream. 
• Great way to reach potential donors that are unlikely to actually attend your auction.
• Creates global exposure for your organization.

When used to supplement to an existing live and silent auction it can be a fantastic revenue stream.  If your target online audience is located out of town(grandparents, aunts, uncles, alumni, etc) be sure to offer things that will be appealing to those folks.