I personally think DrChrono is going for a diminishing slice of the healthcare pie. The current trend in healthcare is that a decreasing proportion of physicians are working in small private practices - the cost of healthcare infrastructure, logistics, and billings means large practices and hospital based have significant savings on scale and tremendous bargaining power with insurance and etc. This kind of app, with the appointment scheduler and prescription system, seems very much geared to small private practices.
I imagine this kind of system will make limited leeway in large healthcare practices and large academic institutions - this is the difference between enterprise software and web applications. Epic and the like, despite being aesthetically unpleasing and rather cumbersome, is dominating this segment of the market. It's not even a question of cost - these solutions cost much more than competitors, but there is a perceived level of stability and a proven track-record. In both culture and priorities, I think YC companies are not as well prepared to offer enterprise solutions. Large institutions are fickle, full of politics, and without a dedicated sales team, difficult to sell to.
They are definitely going for the "meaningful use" incentives as part of healthcare reform - the healthcare reform gives physicians incentives to adopt an EHR (electronic health record) system and going for the sex appeal of using ipads - but I am wondering if that is enough incentive to adopt an electronic health system. Personally, I type 1000x faster on a keyboard than with an iPad, and speed would be an consideration - ie. even as a physician-in-training (hypothetically with comfort in technology), I would have to look hard to decide whether to use such a system.
As a medical student, don't think there is enough of an market for general primary care physicians and not enough specificity (currently) to most subspecialities. There are a lot of older physicians that have limited comfort in medicine. I would imagine there is a good market for concierge medicine practices - the kind of places that already have iPads and are interested in distinguishing themselves from the market.