Well, I would like to answer the question in two ways. First, I would not buy an aluminum waterblock on a high-end CPU. On a cooler CPU with less wattage (such as a small multimedia box like the Shuttle SS51) I might buy aluminum. It depends upon the price differential and also on the maintenance differential. If Aluminum is 1/2 the price.... OK. If Aluminum is much easier to maintain (because you don't have to change the water as often because of less galvanic corrosion), then yes.
Having said that, I would like to follow up by saying that I made my first three waterblocks in aluminum. First, I got the aluminum for free (scrap from the machine shop here at work). Second, I don't know how to machine very well and aluminum is MUCH more forgiving. If you are doing it yourself, learn on aluminum, then join the big boys.
Maybe when I get some free time I'll make my own copper waterblock. Too many pots on the stove right now. I certainly won't buy one if I can make it though, and I could.
As for making it a consumer product, unless you can show a considerable price differential (and I don't believe you can), stick with copper. Watercooling hobbyists are performance junkies and second-rate solutions just won't fly.