"Economically, you can think of a startup as a way to compress your whole working life into a few years. Instead of working at a low intensity for forty years, you work as hard as you possibly can for four. This pays especially well in technology, where you earn a premium for working fast." - Paul Graham
Exactly...one of my recent ventures was spending evenings and weekends for a few months building a prototype HR compensation system for a big company I had done consulting with for years, we had a very good relationship.
All the requirements were met and the hard problems were solved (lots of UI work remained). When it came to decision time, they decided to develop in house instead of licensing my product for $50k. How much did they spend in house? $1 million plus (and counting). Plus, they missed their launch date so have to use the old solution for another year. I could have made the deadline easily.
The point is, the risk of failure is not just that your product is not good enough, it is that the people that make the decisions are often, if not usually, simply not qualified to be serving in the positions they hold.
Targeting individual consumers vs privately owned businesses vs publicly traded corporations are three entirely different situations. If I had to recommend one, I would target the privately owned business sector - there are less developers targeting this market (as it requires domain knowledge), and they actually care about ROI, whereas with a corporation, they are often more concerned about the appearance of ROI.
It's failure if the energy spent on the project is more than the return. Sure you may learn something, but if all you learned was to become a better web developer at the expense of relationships, physical health, your bank account, etc, then perhaps it would have been better to just create a casual, hobby webapp for fun.
By learning I meant learning every aspect of launching a startup, including customer development, sales, marketing, SEO, etc. If your role in the startup was limited to programming, then I'd take a very jaundiced view of the state of affairs.
And by no means would sacrificing relationships and health compensate for even this.
http://www.paulgraham.com/wealth.html