Thanks for the comments guys. I had to disable it again because this little server just couldn’t keep up.
Mark, you’re right. It’s the user experience that’s important. I found that browsing my own blog was slightly slower because the compressed output came down in one spurt. The uncompressed version appears as it’s generated by WordPress and the user sees a gradually loading page instead of “Waiting for…” in their status bar. On a fast server it pays to compress, but here it feels slower.
CPU usage looks about the same on the graph provided by my linode, but when I’m logged in through ssh my session feels more responsive when compression is disabled.

If i have to buy more bandwidth that’s a good problem to have!