This review is dealing with the J! 1.6 version, not the J! 1.5 version.
It works mostly as expected. I like that it uses Joomla's built in mootools instead of trying to include some other JS framework. Most of the meat of the module is in the template file, which is good since most people will want to override the default template to make the module fit their site's design.
It biggest issue with this module is the developer hasn't fixed the fatal bug in the logout form (a missing token) which renders the module useless out of the box. The nature of the bug indicates the developer hasn't tested their module on J!1.6. The fact that they were informed of the bug a month and a half before this review and haven't fixed it yet indicates to me that they are not offering any level of support for this extension.
So, if you're a developer who just wants a starting off point for an inline-popup login/register module than this may get you what you want. If you don't have any technical experience then I'd be wary of the J!1.6 version.