My current house has an interesting mix. A furnace original to the house (1953 Lennox), and a tankless gas water heater, that I believe is less than 5 years old (I've had the house for 2 1/2 years) I can't wait to get rid of the furnace, but replacing windows, storm doors, and putting a new roof on are delaying that upgrade.
I haven't had any issues with running out of hot water when running laundry, dishwasher, and shower - but I also have the heater set pretty high, since it's just Me and the wife. The only real downside is that since it is tankless, you have to wait a bit longer for the water to get to temp, but otherwise I highly recommend them.