Being a plumber I have looked into the pros and cons of these devices and found a lot of cons. First of all is the price , the average tankless hwh is about $1,500 on up. A typical installation or up grade from a tank style hwh, with new gas main to electrical to venting could run ya about another $1,500 to $2,000 . Now all this plus the fact that a single tankless hwh might not give you the volume necessary to supply your home is a VERY large con. I live in the North East and our average winter water temp drops down to around 55 to 60 degrees. For a tankless to heat the water it has to restrict the flow rate to heat the water to the set temp, giving you less hot water. Yes, you will have endless hot water at 1 shower but you better not run the washing machine or the dishwasher, you could see a serious drop in pressure.
A typical tankless will run at a max of 8 gallons per minute. Some high end shower valves with body spray systems can deliver 12 gallons per min, a single tankless just can't keep up. When designing a home plumbing system you have to account for max usage, just about all fixtures running simultaneously. Some of the homes that I have plumbed have 5 full baths, kitchen with 2 sinks, laundry room with slop sink, etc,etc,etc...
Now don't get me wrong these things have their place. If you live in a small home with 1 bathroom or a condo, these things are perfect. But if you live in the northern half of the continent in the average new home of 2,500 sq feet or more with 3 or more baths then I would think they aren't for you. Unless you have about 10 grand to lay out for an elaborate system of these things then more power to ya. The average 3,000 sq ft home in my area would cost about $5,000 to upgrade from a typical 75 gallon tank style to 2 tankless hwh's.
Now a tankless warranty is 20 years(prorated) opposed to 10 for a tank style. 75 gallon tank style $2,000. One time upgrade to 2 tankless hwh's $5,000. Say in 20 years you need to replace the tankless ones at a price of $3,000, you have spent $8,000. In 20 years with a tank style you will have spent $6,000 and did I forget to say that for $2,000 you get a power vent hwh which requires no pilot. Yes, you will save some money on fuel but the cons definitely outweigh the pros.
Check out this link and compare for yourself, scroll down to the chart around the middle of the page.
http://www.aceee.org/consumerguide/waterheating.htm