My experience was relatively short. I have major depressive disorder and a generalized anxiety disorder, both of which manifested themselves while I was in the MTC. When I returned home after eight weeks, several people wouldn't speak to me anymore, and multiple people in my home ward assumed that I'd committed some kind of sin that would keep me from the mission field.
I loved my mission! Even though I have since become disaffected with the church, I still have a lot of fond memories from my mission, and I have kept all my journals, photos, and some memorabilia. It was on my mission where I met my wife. We were both missionaries, where I was her district leader. Even though we didn't have feelings for each other on the mission, her mom invited me to her homecoming, and we started dating, and the rest is history. But I wouldn't trade my mission experience for anything. I served a lot of people, did a lot of great work, and overall just had a very rewarding and fun time in Toronto, Canada.
I didn't enjoy the pressure and expectations of serving a mission as I grew up, I always had the feeling that I didn't want to serve a mission. My dad hadn't gone in a mission, and even though I was always told it was because they couldn't afford it, it showed that you could still be good and not serve a mission. I was very uninterested in church and the doctrine well into my late teens. There was a growing anxiety about what would happen when I turned 19. I finally told my parents that I did not plan on going on a mission, and when they demanded to know why, I had to admit it was because I didn't have a testimony. I opted out of BYU too, choosing to go to a school closer to home (UGA) so I could be closer to my non-member friends (I'd started playing guitar in a garage band and we all wanted to go to college together and "make it big").
My dad had a heart to heart one day bringing up my lack of testimony and said that he found his testimony to be a "pretty big deal". He said that if all the church stuff was true, then it'd be pretty important to figure it out. That it might just be the most important thing I figured out in my life. This piqued my interest and I remember rebelliously thinking I could investigate the church and then after I concluded it wasn't for me, I could share that with him and move on with my life confidently and peacefully. So, I studied to understand what the big deal was. I started going to institute classes, I took missionary lessons, I read lots of scriptures. I got to the point that I wanted to "know". So I prayed long and hard over a period of a couple months. One night during some deep praying, I felt a presence like I hadn't before. I was nearly sure there was an angel in my room and I seemed to feel God's love for me. I didn't turn around to check if there was an angel, I just sat in the love for a few minutes and three feeling faded. Nothing miraculous happened, but it felt providential. I interpreted this experience as a witness from God and though this witness, I had found a testimony.
So ironically, by trying to get out of a mission, I found a testimony and then felt that since I did think it was true, what better way to spend my time than to share that with others. I had to eat crow and tell my parents that I actually did want to serve a mission. I "broke up the band" and left my friends to serve God. To their credit they all respected me for it too.
I went on a mission because I loved people and wanted to help them. I am so glad that I had my own reasons for going and didn't go out of obligation or expectation. Many I served with and around had no real interest in being there and didn't have a nice "conversion" story to share. I do think it made a huge difference in my attitude while serving though (and after my mission too). I served a mission in France and Belgium and really enjoyed myself. It was a so-called "hard" mission because we didn't baptize many, but that was ok with me. I enjoyed serving in foreign lands and learning a new language and culture. I didn't follow sales tactics I was taught and expected to use, because I respected people and let them make their own decisions. I didn't record my teaching statistics and when was asked I would give a number that generally represented the week. I did train new missionaries and was called as a district leader multiple times, so don't think I didn't try my hardest to really do the work. I saw one baptism on my mission. A woman who was suicidal when we met her and we helped her turn her life around - it was such a positive experience for all of us and I'm proud to have helped her. I am fairly certain she is no longer active in the church, so my mission had a net-zero effect on actual church membership, but I know I made a difference in her life. I made great friends on my mission too, something I never expected. Many members from areas where I served as well as fellow missionaries are among my closest friends.