I've been using JIRA::Client since around v0.11 and it's been a great help in many areas of JIRA, especially in migrating from a proprietary bug tracker to JIRA where I've had to script quite a lot of things.
Early on, the module had some pretty big functionality holes, but they've been consistently plugged by the author, usually going beyond my feature requests and suggestions.
From an interface point of view, I rather like the AUTOLOAD approach and the implementation has pretty much done the right thing and the helper methods are a great addition. From a tidy-up point of view, some accessor methods for the various object types returned from the server would be nice rather than using the hash refs, but that's just sugar.
The documentation is getting a lot better, but I've also struggled from time-to-time with some of the unfamiliar interfaces to JIRA's SOAP client. Generally though, I've managed to guess the right thing, often helped by the module's design, and the author has provided abstractions when I have suggested them.