We used to disable compression on links over 128kb. With the 2651 you could perhaps compress up to 256k. Beyond that, the profits become marginal and your CPU-load is raising above average.
On low-speed lines (=<128k) I have much better experiences with TCP header compression, but this implies that you are using TCP which is likely, but not necessarily the case.
At speeds over 512k, I would advise anyone to forget about compression.
