For competition, I inject brisket and butts with Butchers (they have products for both beef and pork). I mix it 1/2 as strong as the package recommends. Both compliment the natural meat flavor beautifully. Fab is a similar product. Both are available on the net.
When I do whole chickens, I inject with a mixture of extra virgin olive oil, garlic powder, onion powder, and a little kosher salt. This helps the breast meat stay moist and helps the bland chicken flavor out a bit.
Injections are primarily used to maintain moisture and promote good texture and secondarily to add any flavor. I appreciate Cook's comment about the natural meat flavor and, when I inject, just try to enhance it, not change it.