This is gonna be kinda long. I've been lurking for a while and figured I could help.
Some background: In January of this year, I had been on finasteride for about 2 years and I was on minoxidil liquid/foam for about a year and 3 months. I was about a Norwood 1.5 at that time and my hair was pretty long - around shoulder length.
My hair loss seemed to have stopped at a solid NW2 after a year on finasteride, but still had lots of hair in the NW1 zone. I figure this would be considered an NW1.5, so that's what I'll call it. The right side of my hairline was in worse shape than the left, so I thought minoxidil could help me regrow the lost hairs on my hairline and get me back to a thick NW1 and/or even the hairline out. I wasn't one of those lucky guys who get a ton of regrowth on finasteride, sadly.
After a few months on the liquid, I switched to the foam simply because it was easier to apply and less greasy. I remember a pretty serious shed around that time, but I didn't think too much of it. My hair was pretty long and you couldn't really tell I was losing hair. For the year I was on the foam, I kept losing more and more hair. I was even dermarolling pretty hard with a 1.5mm roller every couple of weeks because I read that was supposed to be more effective than minoxidil alone. However, the hair loss kept progressing. All the hairs in that zone between NW1 and NW2 were thinning. The finasteride seemed to have been doing its job and holding my real solid hairline at NW2. However, everywhere i had applied minoxidil (the NW1 area), the existing hairs were getting thinner and thinner. I also started noticing heavy bags under my eyes like I had never experienced before. Even with a night cream, it looked like I hadn't slept in weeks. Sadly, things didn't seem to be getting better for my hair, so in early January, I stopped the minoxidil. Things got pretty bad at that point. It felt like minoxidil was making things worse, even after 1 year! I had enough, so I decided to shave my head. A huge decision, but I think it was the right choice because it just looked stupid.
So, I've been off the minoxidil for about 5 months now. There was a period of super heavy shedding after about a month off the medication. Only just now am I starting to see some recovery. I'm hopeful that some alternative treatments I'm on will help me, but that will take a little while to really know if they're truly working.
However, to your question: should you quit minoxidil? If you aren't getting any skin issues, I say no. At first, I was like you. I really didn't think minoxidil was doing anything for me. I really believed that it was making my hair worse the whole time I was on it. However, after couple weeks after I shaved my head and stopped minoxidil, I took two very high quality pictures of my hairline with flash (before any shedding occurred due to stopping minoxidil). My hair was a centimeter or so long at that point, but I was amazed at how thick it really looked. I could see a ton of velus hairs in that photo at the NW1 zone even on the worse side. I took a similar photo of my hairline now and compared them. The minoxidil shot was thicker and there were a lot more terminal and velus hairs. There was marked improvement and I think if I stayed on minoxidil for another couple months and suffered through another hair growth cycle, my hairline would have been even better. I lost a lot of hair in the NW1/NW2 zone after stopping minoxidil BECAUSE minoxidil was working.
Do I regret stopping? Surprisingly, no. My hair may be worse now, but it's starting to recover slowly. The skin issues it was giving me were also dealbreaker for me. I already look older than my age and my skin has always been pretty bad. My skin was just horrible on minoxidil. However, if it didn't give me skin trouble, I would have stayed on it for a few more months before giving up on it. Get some really high quality photos with your phone or a good camera (using flash) of the left, right, and middle of your hairline. In three months, do the same and compare them. It might surprise you. That will give you your answer of whether to quit or not.
I think in a couple months I'll really see how much has changed since I've been off of minoxidil. I've read quite a few stories on this and other forums about people really only 'recovering' from minoxidil after about 8 months or so. Basically, the hair cycles should come back to normal and your 'true' hairline/crown/<wherever you applied minoxidil> should show within 6-8 months after quitting. However, as someone in the middle of that time frame, it's kind of demoralizing. I've kept up shaving my head and, when I'm too lazy to, I'm wearing a hat. It sucks, but hair growth is a slow process. However, my skin is noticeably improved and I'm on some alternative treatments that seem promising. We'll see.
I hope this helps.