I noticed that over on Loosely Coupled, Phil Wainewright has included Artix in his list of top ESBs. That's cool, especially given that it appears that he made his choices based on seeing actual products shipping to actual customers. Customers who use Artix are generally quite happy with it.
It's also good that Phil so clearly explains the fact that the term "ESB" is somewhat loose, since I'm not sure I would describe Artix as just an ESB. The "bus" part of the term ESB seems to imply a single transport and protocol, but Artix is carefully and uniquely designed to focus service/application interactions at the WSDL logical contract level rather than at the WSDL physical binding level. And keep in mind that WSDL does not always imply SOAP. In other words, you write your app to the service contract, rather than instantly making it obsolete by tying it to particular transports, protocols, or message formats, and instead let multi-transport-multi-protocol-multi-format-but-still-fast-and-efficient Artix worry about the best way to bind and communicate.