In my view the key and perhaps the key (if not only) reason to decide on angel investing in a company is the entrepreneur. (Often there is not much else you can go with in any case).
Entrepreneur's themselves are driven differently, some by a quest for wealth, others by a desire to control their work and lifestyle and some are driven by a dream that want to realize. Each will have different benchmark for success and a different time horizon (in general it increases as you go down the list).
If one sets about joining these men and women in their journey's its important to know your own expectations and tune in with the the type you resonate the most.
What Value Do Angels Add?
Another key aspect is to know what value you bring to the venture, if your only value is money you would be best advised to stick to other investment avenues (in everyone's interest!). While there may be functional and domain values you bring owing to your past work, there is one inherent value most financially successful people (if you are Angel investing I would assume you belong to this breed) have.
As an entrepreneur I know the passion and love that blinds one to the fact that the RoCE is just not happening and then like an addict one extends the time frame by yet another quarter/year. And perhaps the best value an Angel investor can bring to the venture is to facilitate that the venture and its key leaders develop the discipline to respect measurable value creation in agreed time-lines (in the first place perhaps temper typical start-up exuberance in setting unrealistic expectations).
A Fine Balancing Act
Finally, its the ability to walk the fine balance between being a uninvolved investor (Financial Investor as they are often called) and becoming the entrepreneur yourself. Whether you invest and how much you spread yourself is really a factor of what kind of entrepreneurs you pick and the value you bring to the table, rather than a simple do or don't rule.