I'm using it at work on a daily basis almost. Their expense + flight / hotel booking system is surprisingly OK relative to enterprise software, they also have a mobile app which is pretty great. It's quite a complex topic, lot's of corporate policies to enforce, and basically they had to develop all the tech facade that Kayak, Expedia etc have (flight and hotel booking) as well as a lot of financial logic for expense…
Enterprise Software requires more much investment in Sales/Marketing so these companies often they invest a lot less in engineering.
YC companies historically tend to be more engineering-focused. Many are now going up market so you start to see more YC job posts for marketing and sales positions now so this is probably changing.