A couple advantages for ANTLR:
- can output parsers in various languages - Java not required for running the generated parser.
- Awesome GUI makes grammar debugging easy (e.g. you can see the generated AST's right in the GUI, no extra tools required)
- Generated code is actually human-readable (it's one of the goals of ANTLR) and the fact that it generates LL parsers surely helps in this regard.
- definition of terminals is context-free as well (as opposed to regex in (f)lex) - thus permitting, for instance, the definition of terminals containing properly-closed parentheses
My .02$
