15 February 2005 update
Given my small test sample, I'd be hard pressed to categorically say that one product supports serving more USB devices than another. With different devices (or even USB driver versions) your experience could be different than mine. What is certain is that the Keyspan USB server currently has two advantages over the SX-1000U: the ability to automatically switch sharing a USB printer among multiple hosts and its support for both Windows and Mac OS X operating systems.
One thing that shouldn't be a deciding factor is the SX-1000U's single USB port, given that you can pick up a USB 1.1 hub in just about any physical format you'd like for under $10. With pricing at time of review of $99 US list for the Silex and $97 - $132 on-line street for the Keyspan, I call it a draw for pricing competition.
A lot can be said for the benefits brought about by new technologies in the networking arena, and USB Servers are just another of many new technologies to come. If you need to share a USB MFP (Multi-Function Printer) or just a scanner, I can safely recommend using a USB Server today.
But I personally am going to wait a few months until the technology matures further. As soon as Silex and KeySpan (or new competitors) develop a way to create a 1:n device-to-host configuration, I have no doubt that these devices will become increasingly popular.