Like Flex, you describe your app's front-end in an XML-based language that describes the layout, controls, and behaviors. Also like Flex you eventually compile to a Flash SWF file, so your resulting app will work on any Flash-capable browser (which is pretty-much everything now). Neatest new addition (and unlike Flex) is that you can compile to DHTML as a back-end rather than Flash, so if your client requires a non-Flash target, you're still good.
Check out the link above and try out the LZPIX application in both Flash and DHTML. IMHO, the Flash version is just a bit smoother, but it's amazing how good the non-Flash version looks and feels.
Also check out the Dashboard demo (lower left corner) to see how far the model can be taken. Maybe it's just me, but I haven't seen any Ajax stuff that looks and works this well.
Also check out Cooqy, which is a Laszlo-built browser for eBay.
Finally, check out Laszlo In Ten Minutes. It lets you compile and run stuff online.