I've owned this model for about 5 years now, after buying it to replace a worn out 40 gallon tank.  Buyer beware...the $500 purchase price does not reflect the full price to get this unit up and running.  You can offset many of the costs if you're handy.  Otherwise you'll pay though the nose for licensed contractors.  And the guys recommended by Bosch are crazy expensive.
If you're upgrading from a tank, you will almost certainly require the following modifications: 1)  Adequate ventilation in the closet.  This could mean modifications to the attic, closet walls, and door. 2)  Adequate exhaust.  You will need 6' dual lined exhaust.  This beast puts out a ton of heat and carbon monoxide when it's running. 3)  Gas Supply.  This could be a simple as exchanging the connecting hose from the supply line to the unit, but you'll need at least a 1/2 inch line. 4)  Moving the water lines.  Tanks feed from the top, this unit feeds from the bottom.  I had to cut back pipe in my wall and sweat new connections.
After all that, this unit will not 'just work' without tweaking.  Getting the temperature set correctly is a pain.  You'll constantly battle this, especially if you live in warmer regions where the ground temperature isn't always 55.  This includes every southern state in the US.
I live in Texas, and I finally had to remove the flow control regulator to get things set right.  The major drawback when you do this is that you have NO control over the water temperature.  It heats to full temp, no matter what.  It's also a pain when you have to mix in too much cold water to get a decent temp...it restricts the hot flow enough to shut off the water heater, so you'll constantly go back and forth between burning and shrinkage.  The best way to avoid this is to turn on the hot water at the sink, thereby allowing for a sufficient water flow through the heater.  Efficient?  I think not.  That's several gallons of perfectly fine heated water.  Money down the drain, literally.
Maintenance is pretty easy.  You'll need to de-scale the heat exchanger every couple of years for exhaust to properly vent.  I just spray it down with muriatic acid, and it shines up like a new penny.  You can also give it a vinegar bath.
Would I recommend this?  No.  Would I buy another one?  No.Will I replace this one?  No.  I feel like I've spent too much time on this thing to give up now.  It's like that black sheep of the family...one more trip to rehab and we'll be ok.  I just can't let it go.

Recommended: No