I've been hammering the 1.4 beta, putting approximately 40 hours a week in on it. Been focusing on workflow aspects mostly, so been giving Akai feedback on this where possible.
Beyond the headline 64 bit aspect, I'd say it's definitely got some welcome additions. In the video below, Dan mentions being able to cursor around in Program Edit and change parameters with the data wheel - this may sound fairly minor, but it does actually improve workflow tremendously.
You guys are definitely going to like 1.4, but I don't want to over-hype it, let's just see how it turns out - when its released, we can discuss all the new features in detail.