Well, it is better in many ways, but it is not in others. For example, I like to open my files directly from the file system, and in that case it just does not work the way I want, which is the way all viewers work: they bring up the picture on screen. If you open a specific file with lightroom, it opens in import mode.

But yes, if you start lightroom first, then the viewer is not bad at all.

TO elaborate more - and it really depends on your personal preference - what I do is:

I very often browse through my collection by simply using Windows explorer.
When I do, I have it setup in either thumbnail view or detail, and switching between the two is one click.
In detail view, I have focal length, camera name, date taken, lens, and other detail listed, and I can quiclly sort or narrow down the view by date range, focal length or whatever parameter. I find it quite effective and I don't see myself abandoning this basic but effective way to access my photo collection.
Now, when I want to view a picture full screen, Windows photo viewer is just average. Picasa is just as quick, if not quicker. Way nicer in the way it brings up the picture on screen.
But then, there are times, often, when I want to switch from viewing a single picture, to a more advanced tool for more features. So, from the full screen or windows view of a specific picture, I've been used to hit enter or other 1-click methods to start an more advanced application (I got used to this by ACDSee, years ago, and found this same behavior in mosti viewers for Windows, such as Firesone, Irfanview, etc).
Now, this is just not happening with Picasa. You need to start it from scratch EVEN IF you're viewing an image with PicasaViewer (a separate exe in the sw package).
Of course, Lightroom is worse in this specific regard.
Once I am in the "advanced" image manager, I'd like to have all features of a more complete image manager: tagging, creating dynamic collections, face recognition, possibility to create slideshows, EXIF group editing, advanced searching, etc.

Photo editing is not much of a need, since I use Photoshop for that. Of course, rotating and cropping comes very handy in any case.

So, Lightroom seems to have most of it, but it is the fast viewing that just does not match what I like to do. And Picasa is a bit too buggy.

So, not that I can't live with Picasa, but I am just searching for something "better", from my perspective.