As people are more likely to go out of their way to complain about a product than they are to publicly praise it, an app with an overwhelmingly positive rating is probably a safe bet. When wading through the negative reviews, read carefully for specific grievances. Some may not apply to you, such as incompatibility with a specific accessory you don't own. Others may be irrelevant or sensationalist, such all-caps rants about a specific feature that the developer clearly states is not included. Some could be real warning signs, such as crashed or loss of data. Be sure to differentiate between reviews for the current version and reviews for older versions. This can help you weed out negative reviews for bugs that have already been fixed, or missing features that have been added.
Finding a Trusted Recommendation
As a frequent reader of technology blogs and columns, I've come to trust a handful of experts for their honest reporting, thoughtful insights, and thorough investigating. For app reviews, my favorite is Federico Viticci of MacStories. His site is an excellent collection of editorial, breaking news, and incredibly solid reviews. Whether Federico is comparing a handful of competing apps point-for-point, or diving deeply into every corner of a new release, he's my trusted source of app reviews, and I recommend you explore MacStories.net.
Third-party app directories can also be an invaluable resource. AppAdvice is a great site for reviews, comprehensive lists, and helpful guides. They have a companion app for the site, as well as a guide/catalog called AppStart, which is free for a limited time to celebrate the release of the new iPad.
Understanding the Developer
Learning about a developer is helpful in finding an app that will work for you in the long run. One of the most telling signs of a neglected app is the date of its last update, which can be found on its App Store listing. An app that hasn't been updated in six months or more may miss out on newer iOS features, have unresolved bugs, or worse, may be marginalized or abandoned by the developer. Whether an app is made by a huge entertainment company or an independent developer working from home, frequent updates can indicate that it's more than just a flash in the pan. Marco Arment, the man behind Instapaper, is an excellent example of a developer who's committed to building a long-term solution for his customers.
Also remember that even if an app is brand new, the developer may not be. If the developer has additional apps in the store, there will always be a link to see their other software titles. Companies like Tapbots and The Omni Group, for example, have a bunch of great apps available. Because I trust what they've built in the past, I'd be inclined to try out any future releases.