I retrofitted an instant gas system to my old (very old) house after having had one for years in other places. Time lag between turning on the tap and receiving hot water depends almost entirely on where the unit is situated. In my place, I was able to put it on the outside wall of the main bathroom so the bathroom takes only seconds to get the hot water. The kitchen is further away and takes a little longer. 

In my previous house I had the controllers and loved them. In this place I opted to go without the controllers to save money. You should look at these if your household includes kids though. 

Price-wise, moving from the electric hot water system in this house to the instant gas has resulted in a huge saving. 

One caveat - water pressure can affect the effectiveness of hot water delivery. In my previous houses I had no problems with hot water being used in multiple locations at the same time. Two people showering in different bathrooms would have a good, regular, stable temperature supply of water. In this house, with really low water pressure, its not possible to run two different showers or even run any other hot water while someone is in the shower. So, check your water pressure before deciding.