I fly both shuttles (DCA and LGA) pretty often. A lot of the key points have been hit. B6’s reliability is petty poor, at least from what I’ve seen. Case and point: one of my associates flew down to DC with me last week. I took the 7 or 8 pm AA shuttle, he took the 9 pm B6. I arrived on time, but his departure was delayed until I want to say post midnight, and arrived at 2 am. It was similar on his return. Someone else mentioned the flexibility of hourly flights (except for the evening DCA-BOS gap), and if you gain EXP, upgrades do occur often. Outside of EXP, I think they’re tough given most flights use the A319. FWIW, TSA Pre at terminal B is typically very swift, whereas my limited experience at terminal C has been less than ideal.