I'm absolutely horrible when it comes to sex.
Seriously I have no idea. I have had 8 sex partners willingly (we don't discuss the unwilling ones). 6 of them were in the context of relationships, the other two were my 'slut phase' where I tried casual sex.
None of these men complained or told me 'Jesus Christ you're lousy in bed'. However that doesn't mean I was the best in bed either.
I assume that partners lied, well except number three who told me point blank that the only thing I was good at (good for) was sex. Apparently I was a miserable failure in all other aspects of life.
Mind this is the same relationship that ended with me in casts and sutures and other stuff... so his opinion may not be wholly sane and valid.
"Good" is relative.
Is it "good" that I know how to say "Yes Sir!" with enthusiasm and deep feeling whilst on my knees?
Is it "good" I have hard limits (no scat, blood,scars and no I won't kiss your feet, suck your toes because 'ew')?
I know guys who feel that my minor voyages into Kink was bad, very, very, unforgivably bad. Yet I also know other guys who threw a fit and thought I was the worst possible sex-buddy because I flat refused to take a fist up yonder.
Not gagging has been a problem in the bed department for me, apparently some guys need their partner to choke and gag and make all sorts of distressful sounds in order to feel the sex is good. Others get all distressed and completely lose their erection if you gag a little.
It would be conceited (at best) for me to say 'I'm good in bed'. Not everyone would think this.