My friend has a four or five year old friesian stallion. He is a sweet heart though. I know he nips butts on trail rides, which can be a pain sometimes, but the guy keeps his horse under control for the most part. Other than that there is not a mean bone in his body. HOWEVER, if you have two studs together and a mare in the area, you could have quite a few fights breaking out. They don't even have to see the mare, just be able to ta catch her scent on the wind.
Also, my friend recently had a problem with his stallion mounting a gelding he was keeping on his property. Luckily the gelding had no serious injuries, but he had lots of scratches and was pretty unsightly. Just another thing to think about.
If you feel confident with training a stallion and you have the areas to potentially separate them if need be, then go for it. Just because you keep a horse a stud, doesn't mean it has to be bred, so I don't think that's too much of an issue. Personally, I love the way stallions look, but I don't think I would ever want to keep one.