If you're not quite ready to book, I would recommend trying Hipmunk's new Fare Alerts feature (log-in required). With the alerts set, Hipmunk will notify you immediately when prices that pertain to your search change.
More than just a flight search app, Hipmunk is also good for booking accommodations. The app conveniently offers both a map view and list view of search results, as well as a full menu of filter options. Also, it lets you sort results by different criteria, including Ecstasy, which takes into account factors like price, amenities, and TripAdvisor reviews. Sorting by Ecstasy is meant to help you find the best overnight experience, and I find it particularly useful when searching among unfamiliar hotel brands.
If you're traveling to a new city, you can also try using Hipmunk's Heatmaps, which overlay important neighborhood information on your search results map. For instance, you can use one Heatmap to see where all the bars and clubs are, or another Heatmap to see where all the shopping is. The feature is a nice touch that can help you feel more confident in your decision when booking a hotel room in an unfamiliar neighborhood.
Lastly, Hipmunk also surfaces any relevant "Tonight-only" deals, which let you book last-minute rooms for deep discounts.
As much as I love Hipmunk, I think the app could use a slightly more nimble search interface. For instance, it would be nice to be able to quickly pivot my search between different nearby airports or adjacent travel days without having to start the search over again.
Also, while sites like Kayak and Orbitz offer vacation packages (flights plus accommodations), Hipmunk seems to keep the two separate. It would be great to see these sort of combinations added to the service in the future.
Hipmunk is a reliable performer that's great for finding flights and accommodations. Similar to Kayak, it searches across a number of different travel sources, and it allows you to jump to a booking page straight away. But to me, what truly set Hipmunk apart are its unique features like Heatmaps, Sort by Agony, and Sort by Ecstasy. With these in tow, it's hard not to peg Hipmunk as one of the best travel apps available on a mobile device.