I've tried both one Scriptaculous wrapper and the gwt-fx codebase and I would wholeheartedly recommend the latter. For starters, it feels like things are done "the GWT way": a single EffectPanel to decorate the widget that will be animated, separate effect classes for the most popular effects, sequential and parallel combinations of effects, deferred binding for browser-specific functionality, Listeners and Adapters and much more. Implementing a fade effect took me about 10 lines of Java code. And we all know Java is not the most succinct girl in town, right? Furthermore, while the Scriptaculous solution worked on IE, Firefox and Opera, it would barf on Safari (version 2 if you must know). gwt-fx worked on all of them without any special-casing in my code.
Also, when things don't work as expected, the author, Adam Tacy, is very cooperative and helpful. He has endured a flood of patches from me to solve some problems that I encountered, and when not satisfied with them, he implemented even better solutions after careful consideration and thoughtful reasoning. Just make sure to get the latest version (1.0.0) that has every bug I encountered so far, fixed.
Adding a supported effects library in the main GWT distribution has been discussed a few times on the contributors list, but apparently it is not a high priority task at the moment. For a good reason I might add, since the release of GWT 1.5 is just around the corner, with support for Java 5 and a whole slew of compiler optimizations. However, with the recent introduction of the GWT incubator project it might be appropriate to start trying out some design ideas on such a library and see how it goes. If you care for such an outcome, don't hesitate to let the GWT developers know about it, by joining the discussion in the recent thread on the subject.