I started it as a free MVP about 2 years ago while in Thailand, and given that I was attracting a slow but steady stream of users I decided to build out a commercial v1 from it.
The freemium SaaS went live in March and it's growing monthly. If I still lived in Thailand I would consider it very successful, but I am in the Seattle area now so it's ramen profitable.
The biggest surprise I got was how slow organic growth takes. Every month I gain more users + MRR but discovery seems to be the biggest problem. I tried Google Adwords in June but Google decided to cost me upwards of $5/click for basic keyword targeting so gave that up. I tried Adwords again in November and now google thinks I'm more relevant, so I pay starting at $0.20/click for the same keywords that cost $5/click 6 months previous. I am currently doing experiments to see if the acquisition cost justifies that spend.
From a effort perspective, the SaaS api+backend itself was about 50% of the effort. The subscription service + user dashboard was another 50%.
From a skills perspective, I think doing a SaaS as a solo founder is only practical if you have extremely broad skillsets: Business management, UX, full-stack webdev, devops, sales, marketing, support. Thankfully I have some experience in all those (except sales) so I was able to either do or fake everything required. If you don't have all those skills, you are going to be increasingly reliant on luck, which isn't a winning strategy.