score:6
I've used the FlyAway in both directions twice already. It's very convenient to get downtown reliably. Traffic doesn't seem to affect much the duration of trips (45min seems like a good estimate) as they ride on the freeway in the ExpressLanes, reserved for buses and people who pay for a special pass. My only complaint is that one time, the bus wasn't clearly labeled FlyAway (just a completely black bus), but it stopped near the FlyAway sign at LAX, so I just waived and asked the driver.
There is one tiny constraint to using the FlyAway from LAX to Union Station though, they only take debit/credit cards (no cash), which you pay upon arrival at Union Station. When leaving Union Station, you pay before jumping on the bus.