I wrote a few times to my local MPs ("député", as we call them in France). I usually got a response, though I suspect it was written by their secretary with no other consequence. In one case (related to privacy against surveillance), they raised a question in the congress, which had just a symbolic impact.

It may be different in other countries. In France, Parliament is de-facto a marginal power against a strong executive power. Even the legal terms are symptomatic of this situation: the government submits a "project of law" while MPs submit a "proposal of law" (which, for members of the governing party, is almost always written by the government then endorsed by some loyal MP).