(a) Absolutely. They are practically a necessity in the muggy summers we get here.

(b) We also have never been without one, but our old, noisy one finally quit a few years ago and we upgraded to a Sunpentown (I think ours is the 55 pint one and we got it through Amazon). It has made a difference in likely lowering our electric bill as it works so well, the a/c has noticeably run less, and we are able to set the thermostat at a higher temp, since we got it. Note that the pint total is water pulled out of the air, not the size of the bucket.

(c) Here in the midwest, almost everyone with a house has a dehumidifier and keeps it in the basement - no need to have several all over the house. Ours is in the basement and our house is about 2000 square feet. Works great for the whole house.