> But at the same time I never would have bought the app if I didn't use "free" for several weeks and got the idea it was useful.

There's a middle ground between only having a Paid app and giving a way a full featured, Free, ad-supported version. For example, you can create a Trial version that stops working after a certain amount of time. If you're making a game, you can only have the first few levels in the Free version. If your app creates content, then you can make a Free version that works exactly as the Paid version but does not allow for saving the users work. [I'm assuming these tactics are not forbidden by some EULA or whatever.]

Anyway you get the idea: there are alternatives that let the user see if they think your software is worth buying that don't involve giving the entire thing away and hoping to get enough ad impressions / conversions to make it worth while. Why so many iOS developers go this route instead of the above and just give away their app with some low eCPM ads is beyond me, though I've only dabbled in iOS development so maybe that's just what works for people :)