My views:
The Good:
I've used COAs to sell before on eBay. Like it or not, people trust the opinion of someone they have heard of more than someone they don't know. A name brand COA will help you sell a $200 arrowhead for about $200. I don't think they add much additional value, but they help things sell (which is a value if you want to move rocks.) On high end stuff, or in person transactions, COAs don't do much anymore.
If you send in something that you've had for years, or bought from a random source, you can get a pretty decent opinion on it from any of the major COAers. (They'd have no reason to paper something bad, or kill something good.)
The Bad:
If you need a COA to convince you a point is authentic before you buy it, you'll probably end up with a beautiful collection of modern art.
The Ugly:
If the seller has hundreds of pieces papered by the same COAer, be cautious. Some COAers seem to take the idea that "the customer is always right" when the customer regularly submits hundreds of pieces for COAs. I personally know of a birdstone that one of the big names rejected when a friend sent it in. He got his money back from the seller. I saw the same piece with a paper from the same COAer a couple of years later at a regularly scheduled auction.
I've seen a lot of G-10 pieces with papers sold by the guy who probably carpooled to the show with the COAer. Buyer beware when the COAer profits from a good call beyond his $25 fee. The Walmart example is the documented example, but there are hundreds of other pieces with COAs that make you scratch your head until you hear some of history about where it came from.
