Their flight search engine found a great deal on American Airlines for 50% of the cost of what Expedia or Travelocity could find. It's offered from airfare.com, which seems to be a low-budget travel site with strict cancellation policies. So far so good.
I also found a hotel in Paris for the last night of my trip. It was a decent hotel within my price range, so I picked it. It was offered by PlacesToStay.com, apparently powered by WorldRes. Less than 24 hours later, I received an email from the hotel saying that they are cancelling my reservation because WorldRes has not been paying them on time, but if I'd like to book the hotel directly, I should contact them. That's not a good sign. Granted, it could be that the hotel is trying to side step (pun intended) the commission from WorldRes, but it seems like they could only do this for so long before they were delisted. So my bet is that the hotel is telling the truth and WorldRes is delinquent.
I contacted SideStep and told them about my incident. It's not in their interest to partner with financially irresponsible companies, so I hope that this will improve over time. At present though, I'd say to be cautious whom you book through.