I tried this Indonesian mobile vendor this week. I was disappointed to learn that they had sold out of the special - nasi gudeg, which was billed as chicken/young jackfruit curry, served with sambal belacan and hard boiled egg. I was left to the more standard gringo-friendly stuff, and had the "undecided" combo which included stir fried noodle, salad, 2 chicken satay skewers and curry beef lumpia. Good versions of these items (actually the curry beef was a bit anemic), but I remained wanting for the funk and heat of the more authentic fare.
If have interpreted the bumbu Facebook and Twitter feeds correctly, the specials are consistently interesting, and they also seem to sell out of them quickly. Hopefully they'll adjust this, as Thai Curry Simple did when they began having an interesting daily special instead of only on Saturdays.