I took a look at the Beta 3 of Adobe Lightroom a while back. I've been working with Beta 4 (on Windows) over the past few weeks. My take? Progress, but much distance left to cover and some nagging concerns.
First, to the progress. Performance is significantly improved. I had actually given up on trying to use Beta 3; it was just too slow. (On a middling but respectable Athlon 64 3000+ with 1GB of memory.) While Beta 4 isn't quite a speed demon, it's certainly usable--which tends to alleviate my concerns that the performance issues were something fundamental and architectural. I would hope that performance improves still further by the time the product is released, but at least it's getting into the ballpark now.
Now for the bad news. And, yes, it's still a beta. But a quick perusal of the forums paints a pretty clear picture of a product that still needs a lot of work to improve reliability and squash bugs. However, likewise, I have no reason to believe these won't get fixed by the time the product is released--assuming that Adobe takes the time to do so. Not that I've been using the program heavily, but I have imported several thousand images and used them to process several hundred images for a big flickr upload session. And it's held together for me quite well.
In fact, my bigger concerns are not whether Adobe can get this program working and tuned. I think that's likely. Rather, I still have some of the same questions that I had after I looked at the program the last time. I still have an issue with the way the program essentially treats the photos in a completely separate organizational metaphor and classification scheme than the one on the disk. Given that Lightroom (and Photoshop) certainly won't be the only applications I use, I can't say I really like this approach. Perhaps I'll adapt or otherwise find a comfortable way to slip Lightroom's metaphors and workflow into mine--but that's perhaps the bigger concern for me than the performance and bugs of the moment.