Google Web Toolkit

Pros

Large list of widgets
- form widgets (checkbox, combobox, listbox, etc...)
- menu widgets (flyout menus, trees, etc...)
- other widgets (sortable tables, tabs, dialog boxes, etc...)
Back button support
Familiarity for java developers
Debugging environment similar to java
Hand-write JS in java with JSNI
JavaDocs offer good documentation

Cons

New browser support must come from Google (currently doesn't support IE7)
Non-standard table-based layouts
Written for java 1.4 (can't use generics, annotations, etc...)
Security and performance issues unknown
Unproven technology (no existing large-scale projects)
Auto-generated JS difficult to debug

Differentiators

Back button support for AJAX
Robust widget library
Java skills required
Unknown, unproven
Auto-generated JS difficult to debug
Not web standards compliant

DEMO: http://code.google.com/webtoolkit/documentation/examples/

Javascript Library Reviews