I always try to answer the following questions:If I had no...

I always try to answer the following questions:
If I had no investment at all in the company, would I find the current / SPP price attractive?
Is this the best place I could put the spare money available for it?

If the answers are "yes", and if the market price is near the SPP price I would always participate in the SPP. Buying shares on the market contributes nothing toward supporting my investment, whereas the SPP strengthens the company I'm investing in.