It's well known in geek circles that the iPhone App Store doesn't allow "try before you buy" distribution. It's less well known that app sales have been less than some had hoped.
I think slow sales and the lack of demo versions are connected.
I "terminate with cause" at least 75% of the desktop software I try -- and I only try products that I want to buy. In most cases the software is either seriously buggy, or it fails a critical test (such as the ability to export and import data).
Reviews should help with this, but they don't. It's not just that reviewers need to be kind to keep getting software, it's also that readers don't like negative reviews. Illogical, sure, but this is humanity we're talking about. We're hard wired to mix the state of the product with the state of the reviewer.
I'm not just making this up! I've been writing Amazon reviews for many years. My positive reviews are always more highly rated. Sure, it could be a retailer rating effect, but my recollection is this effect has been seen in cognitive psychology studies as well.
This human glitch means that a rigorous software reviewer would soon lack for readers. Even amateur reviewers generally like to have an audience, so those that survive learn to be gentle.
The inevitably weak state of the product review marketplace, and, yes Andrew, the fact that I push the limits of software, means I have to test personally. The App Store doesn't allow this. So geeks like me are slow to buy, and that means we're slow to talk about the software. Even if we're few in number, lack of geek chatter impacts sales.
There's an obvious solution.
The App Store should show two buttons for every item. One is "demo", it downloads the demo version. The other is "buy". The demo version would follow the usual practices of desktop demo software: limited lifespan, some carefully chosen feature limitations, use of watermarks etc.
I expect Apple will do something like this soon (it is kind of obvious, after all). Then App Store sales will improve -- at least for quality products.
Interesting lesson about the limited utility of product reviews however ...