If you are only using a subset of the fields, then the performance difference can be substantial. See the following example, which involves retrieving 1,411,771 rows from CScout's analysis of the Linux code.
$ time sh -c "echo 'select * from IDS' | mysql cslinux >/dev/null"
real 0m5.622s
user 0m2.580s
sys 0m0.532s
$ time sh -c "echo 'select EID from IDS' | mysql cslinux >/dev/null"
real 0m4.492s
user 0m0.716s
sys 0m0.096s
An this is not even measuring the performance impact on the server.
