They aren't used around here. They are really more of a point of use type heater. It depends upon your house and water usage. From what I have heard, to get a tankless that would heat water for more than one shower, washer, or fixture at a time can be more expensive that it is worth. Without the storage tank, you have to wait longer to get the hot water. Not worth it in my opinion but maybe there is someone who has actually used them who can prove me wrong.
Talk to the plumbers and other people in your area to see if you can find someone who has actually used them. There is a tankless guy who hangs around the plumbing forums bragging about tankless but he is also soliciting sales over the internet so you have to add a few grains (or a shakerfull) of salt to his opinions.