I've had good experiences with them. And, they have Linux as well as Windows drivers for their ISA and PCI boards; IIRC they also have Linux drivers for their Ethernet serial hubs. The Si series (e.g. 4Si) serial hubs supposedly can be shared among multiple computers. Oh, the ISA/PCI boards don't require an IRQ, which is very nice.
I currently own a 8-port PCI Rocketport with octopus cable, and a 16-port ISA Rocketport with breakout box. The box is more expensive, but nicer in many ways. I bought both on eBay.
Provantage [link|http://www.provantage.com|http://www.provantage.com] has good prices on new Rocketports and typically ships quickly.
I know Andrew G really likes another brand of serial hubs; IIRC they were Digi.
Tony