Yes, I've written to my MP on several occasions over different issues! I think it's important to speak up and be involved.
I have campaigned against building a new airport near to my parents' house, I have written urging my MP to vote in favour of equal marriage for everyone, I have written about funding for local services such as doctors and schools..... I can't remember all the different issues to be honest. Whatever was a hot topic at the time! The MP is supposed to represent his constituency but he or she can't do that effectively if they don't know what the constituents think on the issues of the day.
If you sit back and are passive, thinking there's no point writing or making your voice heard, then your opinion cannot be taken into consideration and you will struggle to hold your MP to account.
Oh and sometimes I get a thoughtful well-considered response and sometimes just a generic "thank you for your interest". On one occasion, I got a really enthusiastic thank you, as the MP had wanted to raise some queries but found it easier to do this with a letter from a constituent. I felt really proud that time!