All of mine (Razorback musks and a few map species) will eat them with gusto. I think they are decent nutritionally and they are easy to gutload with good stuff as they will eat about anything.
Superworms are nutritionally better than mealworms but a much larger "worm". They are very active and live longer in the larval "worm" stage as opposed to mealworms that need to be refridgerated to slow this process if you have a large amount that needs to last more than a few weeks. The superworms eat voraciously and are easy to gut load. Their exoskeleton is easier to digest and not as hard as the mealworm. So a good choice if your turtle has not problem with the larger size of the older larva. I order them from Rainbow Mealworms and choose the medium size.