It seems like a great way to reduce water usage - convert your old toilet with a dual-flush conversion kit, and choose between a full and half flush depending on the situation.
But saving water might not be that simple, according to a joint cautionary statement released by Veritec Consulting Inc. and Koeller & Company. In fact, it could potentially increase the amount of water used.
The statement list four areas of concern:
The removal and replacement of an original equipment manufacturer's (OEM) flush valve with an after-market product changes the full flush characteristics of the toilet fixture. The delivery of the right amount of water to the bowl at a certain rate is critical to the full removal of all solid and liquid waste from the fixture. This is especially true of 1.6-gallon and 1.28-gallon toilets where the hydraulic characteristics (profile) are finely tuned to achieve the maximum "force" of the water delivered through the flush valve. Changing that profile can adversely affect flush performance, potentially leading to double flushing by the user, thereby increasing water use. In a perfect scenario, dual-flush retrofit flush valves should be flush performance tested with each model of the gravity-fed toilet into which it is to be installed to assure the purchaser or end-user that flushing performance will not be sacrificed. In reality, however, such extensive testing is largely impractical and, therefore, has not occurred.
Similarly, the reduced flush is particularly vulnerable to unsatisfactory performance from a retrofit product because the user expects a complete exchange of water in the bowl when the fixture is flushed. There is no assurance that the after-market product will, in fact, remove all of the waste. That is, if it appears to the user that the reduced flush has not removed all of the liquid waste (and potentially toilet paper), the toilet will be flushed a second or third time.
OEM components internal to the tank not only provide for the efficient evacuation of waste, but also assure that the proper amount of refill water is supplied to the bowl to re-establish a minimum 2- inch (50mm) trap seal, thereby preventing sewer gasses from entering the room. Each model of toilet requires a different volume of refill water to be provided from the fill valve to achieve proper trap seal restoration — adding more water than is required to achieve the trap seal is simply a waste of water. Testing has not been conducted on these dual-flush conversion devices to demonstrate that the appropriate amount of refill water is delivered to the bowl.
Most of the conversion devices are fully adjustable and allow the user to increase the flush volume well above the originally rated volume of the fixture, thereby negating much of the expected "savings".
Of further concern is that most of the devices have not been tested to IAPMO's PS 50-2008 standard to ensure durability, performance and leakage requirements.
A PDF of the statement can be viewed at: http://forms.iapmo.org/docs/dualflush_caution.pdf 
Source: IAPMO Green Newsletter, Aug 2009