Ours isn't quite "instant", it does take a minute or so to warm up. We have a small house, so went for the "single use" size rather than the "whole house" size. I actually grew up in Europe, so was used to having one and HATED not getting a hot shower myself. We also had The Great Walt Disney World Flood of 2003 (our water heater ruptured a few hours before we were leaving for a week-long trip we won from Kellogg's to WDW and flooded our finished/DDs bedrooms/homeschool room/laundry room basement...DISASTROUS). The idea of not having to deal with a ruptured water tank ever again was VERY appealing to us.

Ours is natural gas and small, only a bit bigger than our electrical box. It's on the wall of our basement and hubby installed it (but he's a handy guy). Our tank heater was electric and the month after we removed it, our bill dropped on average from 20-25%. Our gas usage barely saw a blip in an increase. LOVE it.