My friend and I are currently developing an iPhone app, it's very challenging and we may have to outsource the development of some core features, which is going to cost a lot.
the problem is that we still haven't determined if the idea is worth developing.
does it provide a real value to users? are they willing to pay? and how much?
so what we have in mind is to have bloggers(smartphone app reviewers) write about our still in development app and see the blog reaction to it.
is this a good way to find out?(can you recommend a good blogger?)
would you recommend a better way?
To begin with, there is no sure way of testing how strong demand is for a new product. If we had figured that out, then there would be no risk whatsoever in starting your own business.
Why do you need smartphone app reviewers? They may focus on the technical aspects of your application or on small details.
At this point, I think that what you need to do is to figure out whether you have a good idea or not. To that end, I would simply talk to potential end users of my app and figure out if they'd like to use it and even pay for it. Once you I have that information, then I would talk to experts about the technical stuff.
If you need opinion of other people, then you have to ask them.
I doubt that your idea of asking bloggers is good, because:
a) people reviewing iPhone apps review apps, not ideas for apps, so they'll be unlikely to write about your idea
b) you won't get high-quality feedback from blog comments
a) describe the idea to real people, ask them for feedback. Your friends, family, coworkers, complete strangers
b) describe the idea here and ask for feedback. There are other forums that have community that might give you good feedback, like http://discuss.joelonsoftware.com/?biz or http://news.ycombinator.com/ c) describe the idea in a blog post and explicitly ask for feedback
Possibly offer a fee app without some of the core features. Link it to a web portal that describes the full app and see if you receive any inquires asking when the full app will be available. This will tell you if people are interested or not.
Using surveys with incentives is a great way to do some research. You need to send the survey out to the right lists that represent your target audience (you'll need to buy the list(s)).
You can use companies like Marketo or Constant Contact to run your survey. Constant Contact is pretty cheap. You'll probably want to experiment with different incentives and different messaging for your survey to see what resonates well with your audience.
 Win an iPad, win one of ten $50 Amazon gift cards, whatever.