Probably not the feedback you were hoping for as I live in the midwest, not LA, but since no one has responded thought I'd give you my 2 cents. As you probably know, Fi uses T-mobile, Sprint, and US Cellular, picking the best signal but seems to default to T-mo when all are available. I would have to imagine T-mo is very good in LA, they seem to have great coverage in major cities. Sprint seems to have good coverage in most major cities as well, not sure about US Cell but that's what usually covers me when I venture out of Des Moines into more rural areas.
I will be travelling to LA for work sometime in the next couple months, so will report back then if you're still waiting for feedback. I'd say go for it though, I switched from Verizon to Fi 6 months ago and couldn't be happier.