Hmmm...I too have to say that I disapprove of Apple's lack of clarity regarding the inability of an AirPort Express to wirelessly extend a wi-fi network that is broadcast by a non-Apple access point. I'm certainly not a networking expert (in fact I struggle a good deal with how networks work under the hood), but I'm not a complete dope when it comes to configuring computers and associated pieces of kit either. There would be a significant proportion of the population with very little tech know-how who would struggle a lot more than I would with this sort of thing. Yet, I spent several hours trying to do exactly what this thread describes can't be done with an AirPort Express: wirelessly extend a wi-fi network created by a non-Apple access point.
Just last week I bought an AirPort Express to do just that. Thankfully, I bought at a good price on eBay, so I don't feel stung, at least not financially. I do feel let down a little by Apple's wording, which, while is correct, doesn't emphasise enough that the base station has to be Apple in order to to what the OP is trying to do. Yes, while it might seem obvious to some of the posters here who may have a good deal of knowledge about networks and Apple equipment, it's not patently clear to novices. There's no vald reason why Apple shouldn't make it patently clear; they do an excellent job of making so many things clear and simple, but not this particular point.
It's only after reading this thread and a couple of similar ones that I learned that what I've been trying to achieve will not work.
So, I'm left with the choice of using power-line adapters or running an Ethernet cable from my modem downstairs to my upstairs where I can then connect my new AirPort Express and create the recommended roaming network. (As it turns out, the last time I ran a cable from downstairs to upstairs, I left a 'pulling' wire in situ, so now I can simply attach the new Ethernet cable the 'pulling' cable and pull it up from downstairs, through the floorboard cavity to where I need it upstairs. Not everyone has that convenience, I concede.)
And as for the "fanboy" comments and the "I bet you never" read this or that, well, they add nothing to an otherwise very useful and informative thread, and I don't condone them. While they may have been posted lightheartedly, the tone gets lost in a printed message and can unintentionally inflame readers. I do thank the likes of Bob and John who provided good information on this thread and many other threads dealing with similar topics. I hope they continue to share their knowledge with the rest of us. It's accurate and trustworthy knowledge like theirs that makes these forums work.
Right, I'm off to order a reel of Ethernet cable. :-)