It would not be smart to make any reasonably expensive and important purchase without scouting around at the options. Looking at gas tankless water heater reviews is a good idea because you do not want to land your family with a unit that fails to meet your needs - if everyone had to take lukewarm showers in winter, you would be unpopular to say the least.
These days there are reviews everywhere - the Web is packed with opinions and user reviews. Some of these tankless hot water heater reviews need to be approached with caution. For instance a user review might be from someone who did not do their research in the first place, complaining that the unit they bought is not up to the job. But if they had done their research they might never have had the problem...
That is not to say this feedback is no use, just that you need to bring your brains to bear on the information. If you see a concern raised that seems important, try to corroborate it - you can even ask for a written response to the issue from the manufacturer or your supplier.
The ratings tankless hot water heaters are given are essential however. This is the basic research you need to undertake - if the manufacturer says it will do X gallons of heating per minute and it does not, you will be able to get the defective unit replaced. Similarly if you get your installer to quote for your house by coming and looking at it, you have more comeback if it does not do what he or she says it will.