Definitely yes for gas. No experience with electric, but with the price of electricity around here, I probably wouldn't do it.
It sounds to me like a fair bit of the issues people above are claiming are hearsay, screwed up installs or maybe based on information from 3 decades ago.
Altitude, not a problem, every modern heater has settings to control for that generally up to 7500ft+. Most manufacturers offer high-altitude models. I'm at about 5200ft.
Noise? A mild humming should be the only sound coming from it, maybe slightly louder than a desk fan. If it's load or shakes, it's not installed correctly (insufficient air supply, gas pressure/volume, etc.).
Doesn't get hot? My groundwater is ~ 50F and my current unit/house has no issues running full-bore(5.5gpm) at 140F (I suspect it would taper a bit trying to run full-bore at the max temp of 180). Easily supplies enough water for 2+ showers at once. I even installed a hot water spigot on the exterior wall to refill the hot tub(cheaper than using the inboard electric heater to bring up the temp) and to take the edge of the kiddie pool in the summer.
Significant savings over the old waters heaters I replaced, but those were due anyway.
I've installed 3 in different houses so far, and the oldest is going on 18 years. 2 are solely hot water heaters, and 1 is currently acting as boiler (much like MMM's setup). I plan on removing my furnace in the current house and using the water heater for heat and hot water. Probably open loop to start, but debating whether I want to get another unit dedicated to heat, or use an indirect tank.
The ability of everyone in the household to shower/wash dishes/clothes without ever worrying about running out of water is great. I highly doubt I'll ever consider a direct tank heater again.