Crazy stuff. I am just starting a thread about PPC and I seen your post so I thought I would throw my 2 cents in.
First of all use Google Keyword Tool - FREE - just google it. Find out how much competition there is and how much it is per keyword.
If there are a great amount of searches and low cost per click go for it, you will probably make money. You can not control the outcome of someone clicking on your ad. What you can control is how targeted it is.
Example - You are selling acai fire for men. If a girl searches diet help and finds your ad, it probably won't be a sale. Now if you have it set up where men searching for acai fire finds your ad you are way more likely almost guaranteed to make a sale.
I hope this makes sense and helps.
EZ