The performance benefits and the space savings. I've always found it difficult to get additional storage on our SAN, so compression gives me a lot more leeway from a time perspective to get the disk space I need.

One of the things I neglected to mention was that compressed tables are still compressed further as a part of the SQL Server 2008 compressed backup (or third party backup solution), so your backups could actually be an order of magnitude than what they are currently. This is especially useful when you need to refresh development or test environments from those backups (provided that your dev and test are running a SQL Server edition that supports compression).