I have a frend who is based there and he likes it a lot. Travel can be a nightmare with rush hour traffic being chaotic and slow.
It also depends where you will live, proximity to shops or restaurants and what facilities you will have access to and their quality.
Like so many things, it will depend on your initial attitude and whether you are a glass half full or flass half empty person to start with. By the standards of most African cities, Dar has a good reputation from what I hear.
Malaria is big all over the continent, so you would have as much chance of getting it there as you would in Luanda, Malabo or Port Harcourt.