Is this thing trustworthy (from those of you that have used one), I mean does it hold the game information well?
I've had approximately zero problems with mine, not counting stupid mistakes on my part (not having it turned on, etc.

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and if I were to use a 256 meg cart, with multiple images on the cart is the cart menu driven?
Yes.
Also how hard would it be to reprogram say a genesis game to the GBA?
Both systems are quite well-documented, so this comes down to two questions:
1) How good are you at reverse engineering object code?
2) How reasonable/insane is the code to the game you want to port?
There's also the issue of sound - GBA doesn't have a sound generator comparable to the YM2610, so you'd have to find some other way to implement that. I'm not sure how much of a correspondence the GBC sound hardware has with the SN76489 PSG.
Is there any GBA Game creation / programming software out there on the net?
Yes. It seems most people are using a standard ARM-targeted GCC/binutils toolchain, but there are also assemblers that are written specifically for GBA stuff. More info about GBA development in general can be found at
Gameboy Advance Devrs and
GBADev.
Hope this helps.