The eBay ones are good enough but you'll notice almost all the sellers also sell replacement impellors for them... make from that what you will.
The KMB units are £36 each at the current exchange rate and, in terms of quality, are well worth paying the extra £20 each for.
I've a couple of the eBay units and have no problem with them, but if I was putting it in a model that was anything other than a "use every so often for a fun chuckabout" kind of thing then I think I'd rather spend the bit more and get the KMB's
Alex