I tried this out a few weeks ago and (while I really like having OO markup) basically it boils down to this:
1. It runs slow as molasses in winter
2. It requires you to learn its own markup language, which encourages a mixture of style, structure and logic
3. I hate getting tied to a plugin when I don't need it
4. This seems to try to side-step all standards web developers fight for (at least some of us fight for...)
5. Now that we have AJAX-centric toolkits starting to show up on the scene, do we really need something that depends on a closed-source plugin from a single company in order to run something stylish?
6. The more complex the Laszlo app, the more my browser freezes (not exactly running an ancient machine, either)
I may have missed some more of its good points, but it just seems an awful lot to me like OpenLaszlo tries to re-create a browser inside a plugin, within the browser.