Prior to speech, there was media speculation either way and it was not clear for much of the speech what he would say. Masterful management of secrecy of the outcome. Nail-biting speech revealed Musharraf's decision at the very end.
Musharraf had two choices: 1) challenge the impeachment (hard to make stick) 2) resign to avoid a prolonged process - since the incompetent government would have trouble prosecuting (they had not even revealed charges, or defined the process for impeachment)
In my opinion he was less of a problem for the incompetents as President. Now he will be freer to criticize them. I predict he will join or start a political party (most probably with Sheikh Rasheed and others).