My experiences mimic those of 2DoorJK.
I installed a Noritz tankless waterheater (made in the same factory as Rinnai) back in 2007, and used it to supply both hot water to the house, and to the "furnace" (high efficiency fan-coil set-up). Proper set-up is key, as is the water-line distribution to your fixtures (shorter = better).
I initially had a lot of issue with sediment in the line, and this would clog the inlet screen, slowing flow, and reducing the heat output of the unit. Initially I just "fixed" this with frequent (every 1-3 month) cleanings (5 min job). The heater was running for a couple of years without a water softener, and didn't have too many issues, though eventually it did start to scale up. Small pieces would break off, and again clog up the inlet screen. At this point, I opted to install a 1" inline y-filter to help with the sediment, and a Nuvo water treatment system (salt free) to help with the scale. I also descaled the unit at the same time (takes 2 hrs, but is VERY easy to do with vinegar). After that, the unit ran perfectly for years with virtually no further work required. I'd clean out the y-strainer and inlet screen every 3-6 months when I'd swap the furnace filter, and that was about it.
Getting hot water at the tap was pretty quick, but did seem s bit slower than the tank I had previously. I got used to it quick though, especially after realizing it does require a different "method" for best results (IMO). If you want warm water, turn the hot tap on to FULL. When it gets hot, then throttle it down and add cool. The higher the flow requirement through the heater, the faster (and hotter) it will heat (you can use a pump to assist with this also).
When my wife and I moved a couple years ago, we opted to go back to a standard hot water tank based on what the builder wanted to charge for going tankless (it was a $6600 upgrade....bwa ha ha ha). My initial surprise was how long it took to get hot water to the taps. WAY longer than before with my tankless set-up. I think this is certainly testament to the fact that the line routing/length plays a much bigger roll than the heat source. Once funds allow (could be a while since I own a JEEP), I will definitely go tankless again. Smaller, cheaper to run, and just all around awesome.
My 2 cents.