I answered only Speedlight, but I would like to put "intentionally" after my answer.
There have been one or two occasions with my 20D where I have used the on-camera flash when I did not bring a flash because I did not think I would need it or when the batteries in my flash went dead. If I think I am going to need a flash I bring a Speedlight, but now I check to be sure the batteries are charged too.
I carry my old D60 as back up, and the flash on that camera won't even pop up. I don't know how long it has been that way or if I could even get the flash to come up when it was new. The fact that I have never bothered to get it repaired may be an indication of how important I consider it to be.