Is it the one through Gray Line?
My DH and I did this one about a year ago and it was HORRIBLE!!! It wasn't so much the tour as our driver, he was just awful, worst money I have ever spent. Not to mention that it was 87 degrees outside and our bus' AC was not working. The tour wasn't great either, it was mostly driving around in the big bus, the best parts were going to the Santa Monica Pier and the Farmers Market, although we were in each location for maybe an hour not nearly long enough.
I wouldn't recommend them to anyone but you never know you might get a bus with AC and a good driver.