I agree DBDev. If a dev wants his app to be purchased, then, like you said, there should be an option to try the app or outright purchase it. Not say it's Free, then bam you open it up and you really can't use the app without purchasing it, that is annoying.
What bugs me the most though is probably the games. Some games would take you forever to complete, or it's practically impossible to get so far without purchasing something from within the app. Some games have a nice balance whereby the game is free and you can fully complete it without purchasing anything, but if you pay for some IAP's you could complete the game more easily, quickly or get more exciting characters etc then I'm alright with that. But some games are just crippled unless you buy IAP's, which spoils it for me. Just let me buy the game upfront and I'd be happy.
Overall I don't mind them on the whole, as long is the dev is fair with how they implement them. At the end of the day, most devs deserve some money for their time and effort they've put in to produce a decent app or game, but it's when they get sneaky and force IAP's on you, that I don't like. That's when I hit that Uninstall button :wink: