In South AFrica, South African Express have introduced it on the Johannesburg - George route which is +/- 560nm. I'd be guessing here but depending on the winds you could be looking at an increase of 20-30 mins in journey time. There are drivers that post so you can get beter gen from them. I guess there is also the comfort factor of being at FL250 instead of 350 which given some of the wx in SA could make a good deal of difference.
With India and China showing an unprecedented demand for crude oil driving long term prices upwards, the Q400 might carve out a niche.
Rgds
GT