Should I tell you that your startup idea sucks?
Yes.
Yes.
I mean, I hear a lot of startup ideas on this site, and for almost 100% of them I think "that's cool, but I can't imagine ever wanting to use it".
Still, people like me don't make up very much of the market place, so I could think your idea is terrible and be wrong, and I like to see people make things.
There's a phrase I like (although I'm not sure of the source) : "The reasonable man adapts himself to the world, while the unreasonable man attempts to adapt the world to himself. Thus all progress depends on the unreasonable man."
So for almost any startup idea, I'm going to think 'neat idea, but it'll never work and I'll never use it', and for most of them I'll be right, but by being an entrepeneur you've already accepted those odds and you probably have to deal with enough negativity without me adding any. I'd hate to be a cause of fewer cool things being made.
On the general question: there is an obvious distinction to be made between someone asking you for advice on what they plan to spend the next three years doing, and what they're hacking together on a single pizza-fueled weekend. The latter is a learning experience and even without your advice they'll fail fast if they fail, so don't sweat it.