OK. I didn't install it myself, but I do have some numbers to report for those of you considering making this switch to tankless hot water.
-- Installed both the propane and a tankless hot water tank. (Nat gas not avail at our location) at the end of 2011. Taking "tanked" hot water heater off reduced our electric bill by something like 200 KwH per month. (yes, we might have taken shorter showers, we know better now.)
--Propane, averaged over the year has cost about $ 30-40 per month, including tank rental, tax delivery fee, etc.
--Electricity in the Puget Sound, WA area costs about .10 per KwH. (your costs may vary!)
--This means that instead of the Water Heater "paying for itself", it's costing us about $10-20 more per month.(Might be a different story if it was electric, not propane.)
--In addition, a centralized tankless water heater doesn't keep residual hot water in the pipes, so must evacuate all the cold water before the hot water reaches the tap. Even if someone just got out of the shower, you gotta wait. This "cold water sandwich" means a higher water bill and water waste. Some tankless heaters are located right under the sink. This solution would help.
--Here's a bit of support in Michael Bluejay's post here
http://michaelbluejay.com/electricity/tankless.html