You can use it for a load of different methods. You can brine your meat (don't really think that is necessary if you have quality meat). Different breds of pork have different types of fat, meat and skin. If you have a leaner pig, inject with light brine to flavor the meat and keep it from drying out. Personally I do an inject with some spice mixture and rapsöl, a
little egg yolk and water (no idea what it is in English, Canola maybe?) to keep it fluid. You need to yolk to emulgate the grease with the water.. Put it in the blender with your usual rub and inject very small portions on it through the skin in the underlaying fat layer. It will melt away in the fat without seperating your nice bark from the meat, protecting it from drying out.
I don't really eat much poultry, except for a few beer can chickens that lost their way.. Check out the bbq and grill section of the forum : -) it's better than hanging out with all the vegetarians in these parts of the forum (just joking).. But I bet that you can flavour up some poultry with the use of a syringe.
The largest cons of syringes are people that overreact when using it and spray up meat with tons of flavouring. It's like coating a Buffalo wing in a one inch layer of barbecue sauce.. it's just to much.
I personally prefer disposable medical syringes after I found out they are actually incredibally cheap. As you use oils and fats with them, they can be a pain in the ass to thoroughly clean afterwards (toothbrush with your usual dishwashing detergent will work well).. Large syringes with a wide needle will cost you about 1 euro here. Just throw them away and get a new one. Smaller ones are far cheaper, a 20ml plastic one of which you cut the tip of diagonally will work well for skin injecting as well (costs about 10 cents).
And oldsalty.. Always cook if you're competing