Re the tourist tax point, I really wouldn't worry about this. From memory (it was 2 years ago) it was next to nothing-a few hundred CFA maybe as much as 1,000 but as that is only £1 it's no big deal. In fact, we didn't pay it and simply drove past the stop point anyway!
Quite whether it's worth the trip is another point but if you do go head for the small museum at, I think it's called, the Ahmed Baba centre. It is truly extraordinary to see all the beatifully illustrated texts (often medical or scientific) from a time when we weren't even in the middle ages.
Have a good trip!