I've always inadvertently done kegels, but when I found out more about them and how to do them, I started doing them consciously. I don't have a regular schedule for them though. Sometimes I forget. But I try to do the 10 reps at least every day, or every other day, followed by longer holds and gradual releases.
To answer your questions:
1. Yes, I'd say my erections are a little harder, but if I abstain for a few days without orgasming, they are harder anyway.
2. I'm not sure.. I think I ejaculate further, but I feel like I launch more semen out when I flex the PC muscle during ejaculation.
3. I don't think my stamina has improved from kegels alone.. it seems like if I tense up the PC muscle to try to prevent orgasm, more blood rushes into my penis and I feel even more sensitive..
4. That muscle might get tired and possible sore if you overwork it, but that will happen to any muscle in your body. I haven't done that yet, but lately I have noticed that when I hold it (clench and hold), I almost feel like I'm having an orgasm, or at least what it feels like down there when I cum, and this can happen when I'm completely NOT aroused..
Overall, I think kegels are a good thing to do. You definitely become more aware of the muscles in that part of your body. And I'd say it does help with erections.
