Make sure your plumber is experienced with them and has done many installs. You want short supply lines and carefully consider placement of the unit.

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We've had a Rinnai R53 tankless for several years now and love it. We no longer have to juggle dishwashing, clothes washing, and/or showers...it can handle all at once.

It does seem to get "mixed up" during cold weather when I'm taking a shower, cycling hot/cold/hot even if the cold tap is turned completely off. If I go out in the garage and manually adjust the digital temp setting down from 140° where we usually run it, to 104° which is the temp I like my showers, then it works fine. I'm not a plumber so I don't know why that works, it just does.

If I had it to do over again I'd spring for the remote control unit they offered at time of installation so I could digitally set my preferred water temp while showering and not have to fiddle with the hot/cold taps...just leave the cold tap completely off. A quick look on Amazon shows some Rinnai remotes, wonder if they would work with my unit?

Another advantage of tankless water heaters is that they free up space for something else, in my case bicycle parking in the garage.

One selling point of tankless is for energy conservation: to only heat hot water when you need it instead of keeping 50 or more gallons heated 24/7/365... Well, I consider myself to be above average on the eco-conscious scale, but must confess I do sometimes take *longer* showers knowing the hot water is "unlimited," so if you're considering tankless for green reasons everyone in the household will need to be responsible with their hot water usage or it may have the opposite effect.

Overall I'd say go for it!

It is my understanding if not enough water is flowing through it as with a low flow shower head then it does cycle off and on. You can fix by turning on more water and/or keeping water setting below 120. It was illegal where I lived ( due to state energy codes) to set it at 140 so we didn't have that problem very often.

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That's what I thought it was, too, so I experimented by turning the cold water off completely and opening the hot tap all the way. It still ranged from scalding to frigid on an approximately 30 second sine wave cycle. This only happens when outside temp is in the 30's or below. I'd be content with 120° max setting, but my wife wants 140° for better sanitizing in dishwasher.

You should read the owner's manual on your unit and/or talk with a plumber. I am fairly certain there is a minumun amount of gals per minute that have to be going through the unit or it will shut itself off. I noticed it when I turned the shower way low when conditioning my hair. I quickly learned to either have the water all on or all off as the hot/cold cycle it no fun in winter.
Do you have a low flow showerhead?

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It's not "officially" low flow, but occasionally the filter screen clogs with rust to the point that it is likely below the minimum flow specs for the heater. It's not an issue that concerns me, I have solutions, just thought I'd bring it up for hagedl's sake.

Doing a little googling on the topic, I see that the issue is common enough to have earned a moniker: "cold water sandwich." Good one.

Bottom line, even with this minor issue, I'd still go with tankless.

You can eliminate cold water sandwich with a hot loop and a pressure tank installation. The hot loop conserves water and must be insulated Served by a mini pump (look at Grudfos) it provides really rapid hot water at all fixtures. You need a crawlspace and basement for single floor and a chase for second floor installs. Apparently, many building departments do not allow PEX, which makes doing the loop in an existing home easier. Wonder what they do for radiant heat?