I love quail! They are very sweet.
1: Yes, in my opinion. But then, there are cat people and there are dog people.
2:They require as much work as you want to do. They are extremely messy and need to be kept reasonably clean. They also CAN be kept with other birds - they eat the seed that the other birds drop! All they need is their food and their water. They also are social - you can't just get one. Get at least 2 (not a **** and a hen or 2 cocks - either 2 hens or a **** and more than one hen). They do smell a little, depending on how you keep them and how often you clean them out. It's just their poo that smells, not the birds themselves.
3: I fed layer's mash. It's good! They did fine on it. You can give them basically what you would give chickens, as far as I'm aware.
4: Here's the thing! I experimented with this. The stereotype is that quail cannot be tamed or trained because they are stupid. I, however, have successfully trained one male to come when I called him and perch on my shoulder when I was feeding the rest. He was my friend called Earnest, and my boyfriend ate him. But, that is not the point - the point is they are VERY easily tamed if you get youngsters, and VERY easily trained - just give them mealworms as rewards (they take some getting used to the mealworms, but they love the when they do). They are not stupid, and I believe they can be great companions especially since their poo is dry and easily cleaned off the carpet.
5: I only have experience with Japanese/pharoah quail, coturnix japonica. Other quail is beautiful, and I guess similar in temperament, although I loved my white and brown ones.
6: Yeah, you can breed them, but production quail tend to have the instinct to sit on their eggs bred out of them. They just squat and lay and move on. Ask at the pet shop if they sit - if they don't, it'll be an incubator job I'm afraid. Pet quail do not sell for much, so unless you're thinking of building a huge breeding centre and quality quail for food or pets, don't think you'll make any money.
8: Yup! They're great with budgies although better in an aviary than a birdcage.
Another advantage of quail is that they can be kept in quite a small space. And also, you can eat their eggs (yum!)