Living in LA you figure out what works for you. If my trip is short enough I use a corporate rate at the Parking Spot. If not, Uber has always been okay, but recently I found another service called Blacklane. It's not the best option in EVERY city, but in some it's cheaper than taxi/UberX.
I think they offer on-demand now, but it kind of works on a next-day-type of system. You set your pickup and the driver meets you near the baggage claim. They email you when they have your confirmed driver, so you have his contact and car information. You have a standard option (US: Towncar, Europe: E-class, etc...), van (...van...) and Luxury (S-class, etc...).
Taxes and tips are included, and everything is pre-paid. I used it in Paris a few times successfully...really, really happy with it. The drivers were excellent, and when I started having issues with AmEx, one of them offered me his wifi so I could chat with them from my laptop.
The price in Paris was a few euros less than UberX, and the price in LA is very fair ($60-$70 from LAX to Downtown LA). Doesn't hurt to have another option to shop when planning trips (San Francisco is not a city where it makes sense, btw...).