In my limited experience, it seems my cartomizers (Mega 510's made by SGS) have been hit or miss. I got a package of 5, of which 2 tasted terrible no matter what I did. However, the other three are performing like champs over the last several weeks. I've actually stopped refilling carts and have just rotated these three.
The first 5 I got were pre-filled with 24 mg USA Mix which wasn't very good. I've got 5 more empty cartomizers coming this week, so we'll see how that goes. But if you find a couple of good ones, they sure simplify vaping in the car and at work!
Rich