Ask HN: Evaluating app ideas?

Me and a friend have been thinking of multiple ideas for smartphones [and iPad]. I expect that we'll continue coming up with ideas till we have a bunch, and then decide what to work on. We already have a few ideas, but are not sure how to go about evaluating the viability of those ideas [Would a large number of people want it? Would they pay for it? Approximately how much effort would this app be worth? etc]

For example one of the ideas we had is something similar to "Local Chat" for iPhone [ link http://itunes.apple.com/us/app/local-chat-whats-good/id307838489?mt=8 ]. We would definitely want to use the app we want to make, but other than that, how do we objectively evaluate the idea.

Seeking advice and experiences!