We have a table with a little over 4 billion rows that uses SQL Server 2008 Page data compression. With Page data compression, the table takes up 3.8TB. If I were to remove the data compression, we would need about 4.6TB of space for the same table. This is about a 16% space savings (which is not the best I have ever seen), but this is the largest table that I have used data compression on in a production environment.
BTW, this database lives on a little Dell PowerEdge 1950, with one quad-core Xeon 5440 CPU and 32GB of RAM, connected to a 3PAR S400 SAN.