So many people are very quick to complain but are not very good when it comes to giving praise for a job well done. Yes it does help, all letters from passengers are followed up by the company I work for.
A copy of the letter is shown to the crew member concerned and is then placed on their personal file. Depending on the content of the letter commendations can be awarded.
I am sure there are crew members out there who are glad you have filled out your comment cards, so keep on writing!