Ideally? I like 0% better. :) All kidding aside, I think the effectiveness of adwords is very dependent on your niche, the amount of competition and your profit margin.
I'm comfortable shipping a certain percentage of my profit off to Google - as long as it is profit that I wouldn't have had otherwise. I realize there is the fuzzy non-tracked click to sale, but IMHO the vast majority of folks who click thru - if they are going to do so - will consummate the sale on the same computer within the next month. I may be off a little but I think that if they return to your site within the next month with the Google tracking cookie still in their browser then it will be logged as a PPC sale.
I'll give up, say, 1% of my profit for some fuzzy 'branding' idea (no immediate or measurable ROI), but not 10% and certainly not 50%. I can find better uses for that money.
If I was making money from just "traffic" fine. But I make money from sales, not traffic. You can send me a huge amount of crappy or irrelevant traffic - if it doesn't convert then it is worthless to me. One key is to routinely review the traffic you are getting from each SE's PPC program and see who is sending you the most high-quality traffic.
I like AdWords, especially if you have enough thoroughput to bid on CPA instead of PPC - it can be a good deal.