1. Is a gas mask worth having?
what romania said.

2. What use / how practical is it?
only usefull if you have it with you and known when you have to wear it. Also knowning how they work is pretty handy too, every years a couple people die because they forget to take of the seals of the filter cannisters...

3. What features would be desireable in a gas mask?
One that fits properly, hydration tube, optional build in glasses thing, etc.

4. How many cartridges / accessories would be stocked?
What are you preparing for?

5. They are not cheap at 300usd for a top shelf model. Is a cheap one trustable? (I don't climb on poor quality ropes no matter the cost, so guess what I think?)
Many cheap gasmask are wel over date, they may have developed leaks, cracks, etc. Also the canister has reduceded efficientcy.

6. Will it push my girlfriend over the edge or will she kinda like it?
i'm not getting between a couple...