> Is it worth the money?
That's for you to decide. If you are going to do GBA development, it's a hell of lot cheaper (and 100% more available) than an official Nintendo devkit.
>is the GBA language hard to develop with?
GBA coding uses a mixture of C/C++ and the native ARM assembly language. The assemblers/compilers are freely available (e.g. GCC3.0 for linux, ARM SDT for Windows), see www.gbadev.org for more info. Whether it's "hard" depends on how experienced a programmer you are! IMHO, the GBA is a good platform to learn basic games programming (2D-based) on, with a bit of will and hard-work required, of course!
Hope this answers your questions!