I'm going to go against the crowd on this one and say that it's been (I think) worth it for me to at least look at the explanations for every question, even when I knew the answer.
One thing that has really impressed me about UWorld (and that I also liked about BSS and the Robbin's qbook) is that in the explanations they not only go over why the correct answer was correct, but also they tell you what would have been in the stem if the other answer choices had been correct.
For me personally, I often get down to 2-3 choices and have a tough choice between them, but after reading the explanations I feel I'd do better with a similar question in the future now that I know the piece of information that separated my choice from the other, independent of whether I got the answer right or not.
Also, for me the time isn't as big a deal, so that may be clouding my judgment; I can typically get through a set of questions and do the explanations in ~70 mins if I focus, or 90 mins if I take my time/get distracted. For the amount of learning I get, it's worth it.