I see it as a tradeoff in cost/ease of setup vs. specific functionality. You can make a store on Yahoo's shopping thing within a couple of days and it won't cost you much, but you won't be able to customize it much either, beyond changing the basic graphics. The next level is packaged software like Interchange. It will be faster than building your own, if what it does is close enough to what you want. The final level is building your own, probably based on some existing framework like OpenInteract or Apache::PageKit. We built our own at eToys because our needs for specific functionality and scalability exceeded anything that was available at the time. (Note that Interchange may have improved a lot since then.) If you're wondering how much work it takes, read my article about it.
in reply to interchange for e-commerce?