I've found United to be pretty good with rebooking on other airlines. Of course, as suggested always look up flights before calling.
Felt the opposite experience on Delta (rebooked to Delta from United) recently during the winter storm/LGA plane crash. The Delta flight I was supposed to be on was canceled as well. They flat out refused to rebook anyone on US Airways (I was at a US Airways hub) and were providing people options to fly out in 2 days. I even heard Medallion status members complaining about it. I went back to United and they quickly got me on the next US Airways flight out.
Yes, my original United flight was canceled, but the important thing was they were able to reaccommodate me. So, a positive experience in my book
I'm glad I only started travelling 2 years ago. I haven't experienced the "good old days" so don't have such high expectations as everyone else on this forum.