I haven't used openlazlo, but SmartClient is slow on the client. Ditto a lot of the current set of heavy "desktop app" client side frameworks. As soon as you get out of developer class machines things get brutally unusable.
Again, the "make it work" part always strikes me as the challenge, and to me these two projects are several generations ahead of the capabilities of things like jQuery for things like databinding and custom drawing capabilities.