Installed one on the wifes old house 2 years ago.
Ups: continuous hot water. Low energy use, particularly if your uses are sporadic and typical, ie everyone gets up in the morning, showers, runs the dishes, goes to work for 10 hours during which no hot water is used. Small and can be wall mounted up and out of the way. More sophisticated units can take replacements of components of the heater, extending its life for decades. In general, providing the intake water is PH balanced and has no highly corrosive elements, the tankless units will outlast a tank unit without part replacement by at least 20-30%.
Downs: not much savings if you use hot water regularly throughout the day. For gas unit, may have to resize or relocate vent stack and will most likely have to raise it to 4-5' over the roofline to meet many local building codes (suckers get hot). Vent stack might also need to be completely straight with no bends per building code requirements or a special blower fan might need to be installed. The <$500 ones are frequently half inch and will supply hot water to one demand spot and will peter off if you open another faucet. Some people are perfectly okay with this and will crow that they use 3 at a time and its great for them. If you're used to getting really hot water at the sink or for dishes or clothes while someone is in the shower, you wont think its that great. Models that service two or more faucets with good hot water run about a grand. Unit heats water by adding a certain number of degrees/boosting the temp, not usually to a specific output temperature like most tank heaters, so you may have to adjust the burner level between summer and winter gradually. If partial pipe blockage occurs (debris in water supply, corrosion), extremely hot water can be suddenly or gradually produced. Some models require periodic replacement of an intake valve, which is cheap enough, but for some reason the manufacturer used non-stainless screws and the screws cant be backed out after a couple of years, requiring removal and replacement of a much larger portion of the heater - people who have or are interested in installing these units are advised to see if their unit is part of this 'problem', take the heater offline, remove the iron screws and replace with stainless. I believe the cheaper Bosch units and a few others that are similar and sold under different names are the problem children here. Gas units require a tremendous amount of inflow air that is exhausted through the vent stack, so interior installation can be problematic. It'll work but you'll lose a good amount of interior heated air in the winter and cooled air in the summer, and may need to keep a window open or have a special intake vent to an exterior wall installed. Exterior mounting in a garage or external closet is ideal, but if temps in those areas drop below freezing, the radiator in the unit may freeze and burst. Gas models can be very loud, like a small jet engine, making interior installations problematic. If you're used to having a furnace inside your house, this wont be much worse. If intake water temperature is too low, you may either need to use a much bigger unit or put two of them in series.
In our case the implementation was pretty good. Its a temperate region that rarely freezes, we had an exterior well vented closet, the roof was already off the house, so reducting to a larger taller duct wasnt a big deal. One bathroom so we could get away with a smaller unit. Did buy the Bosch and did replace the iron screws on the intake valve with lubed stainless screws. We were also replumbing the house, so any plumbing extras werent extra. Intake water temperature only varied 15-20 degrees between mid summer and mid winter.
Very popular in some parts of the world, where tanks are infrequently seen.
A great idea for people who have big families that need to take an hour or two of showers one after the other, or who have heavy serial hot water usage, or people who use hot water at specific periods of the day with long periods of low/no use.
For a house in mexico, your building codes are likely to be lower than the US, the water input temp is likely to be fairly consistent, and the exterior temps if mounted in an outside cabinet are likely to be favorable. I'd still be careful with the intake and venting on a gas model, regardless of building codes. The one we put in was like a small jet engine mounted to the wall of an exterior closet on the house...