Never used them, but just bought a pack as I usually have half a dozen or so bottles left over after filling a keg. I thought they would be far easier than measuring 4 odd grams of dextrose for each bottle and mucking about pouring it in. I dont think they impact flavour... I believe they are basically dextrose with a binding agent. I wont have much control over CO2 vols but the easy factor wins out.
You won't get much/any flavour impact. Generally a mix of glucose and sucrose. You may get inconsistency in carbonation (not all exactly the same size) and you may find it harder to work out your desired carb level, as opposed to something like bulk priming.
However loads use them, I have too and they are fine for their purpose. I found 2 per longneck slightly too fizzy but I prefer lower carb, generally. Some find 2 too low.*