bzip2 compresses significantly better than gzip (I typically see a delta of between 10% and 30% in filesize) but it comes at a significant cost: compressing/decompressing the same file generally takes many times longer. xz compresses even better but takes even longer.
Also, some web servers (or at least Apache) will serve gzipped static content automagically if it finds a matching gzipped file. For example, if you have /var/www/html/foobar.css.gz, it will serve the contents of that file directly when the client requests http://example.com/foobar.css. So if you're running a stock Apache configuration, there's no reason not to just run gzip on all of your static assets and get lower bandwidth bills right away.
Also, some web servers (or at least Apache) will serve gzipped static content automagically if it finds a matching gzipped file. For example, if you have /var/www/html/foobar.css.gz, it will serve the contents of that file directly when the client requests http://example.com/foobar.css. So if you're running a stock Apache configuration, there's no reason not to just run gzip on all of your static assets and get lower bandwidth bills right away.