I think its hard to give general advice in response to your question "Whats best way to evaluate your idea", but here's a data point for your consideration. Here is how 2 friends and I ended up leaving our jobs to pursue our ideas:
- About 2 years ago, I decided that a major goal in my life was to start a new enterprise that created something new that people (customers / investors / users) would value and would make a difference. I teamed up with 2 other friends who I knew felt the same way and were getting tired of their day jobs in big IT corporates
- We decided to spend time trying to scientifically evaluate one idea with a working title "Travel Friend Finders". Think WAYN or dopplr, but this was before either were released. We thought of business models, and came up with a spreadsheet model which told us that users of the service would not be in the same place often enough to make it useful. So we decided to bin the idea and brainstorm others. Since then WAYN has been launched and is profitable, and dopplr has secured angel funding from the likes of Saul Klein, so this perhaps wasn't a great call or method of evaluating the idea!
- We built a prototype of another service which sent notifications about share price movements via SMS. It worked and we got some trial users, but we gave up on it because we didn't think the market was big enough (after some very cursory market research amongst financial PR firms) and getting real time data on stock was too expensive. This was all evenings and weekends.
- Over time the team changed as peoples' interest waned and other things took priority, but others with the same interest in creating something new took their place. However, it seemed incredibly difficult to get anywhere just working evenings and weekends because, to have an objective basis to evaluate a business idea you need to be out there talking to your potential users and customers. And that's difficult to do on evenings and weekends.
- 3 months ago, I took the plunge and left my job to work on the idea we were considering at the time. I did not leave because of the idea itself, but because I had engineered an opportunity to bring in some money with part-time contracting and realised that the time, money, flexibility and lack of IP assignments that big corporates ask for would put me in a far better position to evaluate and improve our ideas. My colleagues joined me and we secured another contract to bring money in and now it actually feels like we are at the start of something and can make real progress. I took a major cut in income, but it was the best decision I have made in my professional life (my best in personal life was marrying my wife!)
Amir Nathoo