The last time I wrote about tankless water heaters was in 2006 and our current water heater is still going strong.
You could say I'm jinxing it but we have a 9 year tank that's 16 years old. We might be able to get another 4 years out of it or it might go within the next year. Probably best to be prepared and still best to wait it out.
The tankless water heater I'm looking at now is the Takagi T-H3M-DV-N. It's a condensing natural gas version with a 0.93 energy factor, 1/2" gas line, 6.6 gallons per minute flow, and a 15 year lifespan/warranty. It's for indoor use and vents via pvc.
What's new(er) in tankless water heater technology is the fact that they're able to use a 1/2" gas line whereas before you might have needed to upgrade to a 3/4" gas line which would've been an expensive retrofit. The condensing feature is new which drives the efficiency to the higher 0.93 energy factor. Also, the use of pvc for venting and then the whole connecting to a network and managing stuff via smartphone (overkill) are also new.
These newer tankless water heater units are now cheaper and also cheaper to install because of what I've mentioned already with no gas line retrofit and pvc venting. Before, cost to install would be $3,000 – $4,000 and now we're looking at under $1,500.
Takagai, Noritz, and Rinnai are brands that folks seem to like.