I worked with it for quite awhile in preview format. It's VERY fast, but it's not really complete. There's loads of functionality missing from V4, and there's along way to go before there's a real V5, IMO.
I was also in software development for a long time, and they made the classic mistake of biting off far more than they could chew. They rewrote everything, added layers, and added full catalog and DAM at the same time. The catalog and DAM took so much time, that fundamental features such as Perfectly Clear, and the ability to move tools and palettes around, is not there yet.
There were quite a few of us who tried to suggest early on that they can the catalog and DAM, since it's optional anyway, and focus on providing at least the functionality available in V4. It was like talking to a brick wall. If there's one thing I learned about producing off-the-shelf software, it's that once a feature is in, you can't drop it. But they have, so far.
Besides the omissions, there are also a number of known bugs. I think they released now simply because this is the one-year anniversary of their non-release of V5 which they absolutely insisted would be released in 2008, and they were insisting up to late December 2008 that it WOULD be released.
I'm sure they are a very dedicated and competent development team, but what is sorely lacking in their company is project management skills.
Having said all that, their A900 processing is pretty good, and I like their batch facilities and their speed. I wish them well, but I reckon right now survival is their aim. They are so late with the product they have had to issue free upgrades to purchasers who purchased V4 from around July 2006!
EDIT - Here's a few more features that did not get in:
"However, there are a couple major items that Bibble 4 offered that have not yet made it into Bibble 5. These include major tools like Perfectly Clear and the Heal & Clone brush, as well as smaller items, like black/white/gray pickers in the curves tool, Web Gallery output, and others."