The unfortunate thing about apps in mobile apps stores is that trials are often not available, so you have to decide whether or not an app is worth purchasing based on various types of information.
There are reviews, recommendations from web sites (like this one!), the number of downloads, and other information you can find online. But without firsthand experience, you can't really be sure how much you're going to like it. So what's your tipping point? How do you decide if an app's worth buying when you can't always test them out? Let us know your thoughts in the comments.