Just look at these comments.Oh dear I wouldn't want to be one of the developers, imagine the sheer embarassement...
I think the reputation of Jelly comes from Maven, i.e. Maven plug-in writers. Implementing complex plug-ins in XML is a really bad idea. Moreover Maven plug-in API is far from friendly.
Personally I have nothing against jelly, but Maven is a monster;).
I've designed a plug-in API for one commercial product and initially we had only java plug-ins. Then we've added support for XML-based plug-ins and jelly was one of the scripting languages(we've decided to add support for BeanShell too). In my view jelly fits well for creating small sized plug-ins, without dependencies on java specific APIs, especially if plug-in should provide a lot of meta-information in XML form.