This UK site https://www.writetothem.com/ that does a similar job works fantastically. Using it took a significant amount of pain out of working out where to send my emails, what I had to include to be likely to get a response and so on. Even moreso in writing to MEPs. It also allows them to measure responsiveness and publish that.
I like their policy of not allowing templated form emails, on the grounds that it just annoys MPs and they're more likely to reply if they're unique and personal. I also like the general philosophy of "You don't need to know which level of government representative you need to talk to, we'll find all of them and try to help you decide".
To agree with another poster on this thread, in the UK there's also the big problem with MPs getting too many emails to sort through them and reply personally, many from large campaign groups such as 38 Degrees. Democracy Club (http://www.democracyclub.org.uk, I'm a volunteer) also found the same problem in feedback from candidates in the 2015 UK General Election, many having to handle thousands of emails (partially our fault for making them open data ;)) on little to no resources or party support. I think there's definitely a space for a modern product which can group together and help a representative/candidate handle replies to different campaigns. This is much better than the alternative, which is making it harder to contact representatives/candidates.
I think this is great!