Remember that it is an editorship on law review which really carries resume value although law reviews seem to hand out editorships like candy to those who do a second year. My hunch is that the prestige of the journal also figures into the mix.
In my case, I have a problem of being at a very new school. We have no reputation. For me, moot court is valuable because it is an independant yardstick. I am not shy about telling people that we beat teams from BYU and U of Missouri. I think the prestige of the schools we beat rubs off on us.
Q: Is it worth it to participate, since it takes tons of time from other classes?
It probably takes less time than law review and it is less drudgery too. It does take a lot of time. One tip is to try to take a class that covers the topic of the moot court at the same time. Then the time you spend on moot court will also help with the class. (If you are lucky, the class might help with moot court too.)
Also consider whether the skills you gain in moot court might just help you generally improve as a law student. After losing several days of class, a lot of study hours, and, for the nationals, a full week of class three weeks before finals, both my partner and I increased our GPAs and advanced in class rank despite a major tightening of the grading at our school. (Your mileage may vary.)
>Although I'm sure it is hard to measure, does anyone know of firms that
look for moot court experience specifically? Who value moot court over
grades or other accomplishments?
Let's get one thing perfectly clear, no firm values anything over grades or, more specifically, the ability to do legal work as indicated by grades. Law review, moot court, and other activities are things to do in addition to getting good grades because they are extra evidence of your ability to do legal work.
Don't underestimate the value of moot court as a confidence builder. You may find that the moot court experience makes you a better interviewee. I had a crucial interview with a hiring partner who was the ultimate "cold bench." He just had nothing to ask me! If I hadn't had the moot court experience of dealing with a panel of judges that seemed vehemently uninterested in what I was saying, I probably would have folded. As it was, I interviewed him and got the job.