I've been with them about 18 months and I've noticed 1-2 occasions where there seemed to be an outage. Once turned out to be a problem with my ISP. I think the other was their fault, but it didn't last long. Check their twitter feed and site status blog for a sample of the types of outages you can expect.
Within a day, in my experience. They also have online chat-based help which I've used a few times. Things get resolved quite quickly as they can look into you account and test things out. This is a bit frightening in some ways though, because you don't really have any privacy.
I think they don't allow this for security reasons, but they do have a myphpadmin interface. Someone might know better than me on this one.
Just have to look around. You can sign up to be an affiliate before you open an account and you can see the range of discounts available. There are occasional promotions around DH's anniversary where great deals are offered. In the long run, you can save money by paying for 5 years or so at a time because your per-month charge will be a bit less, although not much less.
I've never tried another company other than free hosts, which were a big pain in the ass. I really enjoy the freedom of DH. If you don't abuse their system or break the law, it seems that you can do pretty much anything you want.