I have positive opinion about Instant Geysers, I have installed last year and before switching I was bit hesitated and researched a lot collected some reviews from users and then switched. Geyser which are in the market like Golden Fuji, Extreme, Super Asia, Nasgas I went for Nasgas 8Ltr /per minute and connected to two kitchens and two wash rooms, now after a year no single problem faced hot water supplied instantly to tap within 8 seconds rather in conventional geyser all pipes have cold water and need to wait for flush all. pressure in taps are marginally low but in acceptable range and would not affect system performance however two simultaneous taps usage reduce the performance. if you want to run three simultaneous tap operation then go for 10 ltr model it will suffice. regarding saving its impressive, negligible load on gas bill just consume 300 Rs gas p/m vs Conventional geyser which is approx 4500.
Actually old geyser have larger tank and have to heat-up all water at set temp beside consumption of 1 gallon or 55 gallon then after some time it will again turn on burner automatically without usage so gas bill increase considerably. in instant geyser it turn on burner when detection of water flow matured at load level and stopped when a user close the tap. conventional geyser might be suitable for large setup for contentious operation of all kitchens and wash rooms at good pressure but have hicup when tank emptied after 55 gallon and it will take time to regain heat for next 55 gallons, despite instant geyser can supply you same temperature and flow of water till till main overhead tank has water eg 400gallon.
I have posted my honest opinion some friends have bitter experience, all depend upon environment, quality of product, installation and more important user knowledge about machine. I have noticed some failed installations around me like opening of both taps of shower mixer when output closed which make a reverse cycle and push cold water back from instant geyser and resultantly no other user will get hot water till said mixer identified and one valve closed down.