I find United to be fine, but I think they charge you for the bags because they know they will be delivering them to your house/hotel later.
*snerk!* Love that Marticia.
OP: I will fly United only if they're the only ones available.
Back to United. I flew them coming home from London Heathrow to Chicago and back to Oklahoma. It was raining in Chicago so our two-hour layover turned into four, then five, then eight, then "Oh sorry we're not flying tonight. And since rain is an act of God you don't get a hotel voucher. We were instructed to sleep on the airport floor (ewww!) and given vouchers to fly home on AA the next morning.
Dad was livid and we were both exhausted from dealing with this PLUS the transatlantic flight the night before. I suggested to my Dad we go take our AA vouchers to an AA gate and see if they could do anything for us. Well we found a gate that was Oklahoma-bound and showed them our vouchers. They were loading up a flight headed for our hometown and they had two seats in FIRST CLASS available. The gate lady was happy to let us right on. I was so relieved I burst into tears and hugged her (this was before 9/11 - wouldn't try grabbing hold of anyone now). We couldn't sit together but I crashed for two hours in first class.
Back home, United took over a week to get our luggage back to us.
And no, it was never explained why it was unsafe for United to fly Chicago to Tulsa but American had no problems doing it. Then again ...
Slightly off-topic: the American Airlines repair/maintenance facility is here in my town and those guys REFUSE to fly AA for safety reasons. I flew AA once (about 2004) and the ceiling tiles fell out with a huge BANG that made me and a lot of other people scream. Won't fly AA anymore. They're an accident waiting to happen!