Here is my take on the matter at hand: If you are trying to build a reasonably priced street car with a smaller sized turbo then the eBay kits will be fine. If you're trying to build a racecar or a very high powered car, then yeah, plop down the extra cash for a really nice brand name intercooler setup. I think the larger the turbo/higher the boost the more gains you'll see from a better quality setup.
On a side note, this board sure has been blowing up with drama lately
