Advantages of instant hot water heaters?

Never had one; friend recommended it but seems like it could be an energy hog with lots of power needed at one time. But, is that cheaper in the long run than keeping a full tank always ready?

Pros and cons please; thanks.

 

Beside the drawbacks you already noted, they have a very limited flow capacity. I do have a very small (also cheap) one n my shop for washing my hands, and it works fine for that very limited use, but I sure wouldn't want one for my shower, dishwasher, or clothes washer.