Try searching for reviews on Epinions.com. They have 21 reviews of the Aquastar 125B, for example.

Hi,
In addition to the two mentioned above, Takagi is a well respected brand.

http://www.takagi.com/index.asp

If you ever plan to add solar water heating, be sure to get a tankless that will automatically modulate its flame to add only as much heat as is needed to get the water up to temperature. All three of these brands offer models that do this.

Gary

Here is a link that might be useful: Tankless water heater

Check your gas supply system before investing in a tankless for a retrofit. Many whole house type tankless water heaters require a gas supply nearly equal to your furnace and in many cases the existing gas system is not sufficient to meet the demand.