I strongly recommend the Flavor Injector from
Sur La Table. At $19, it's not the cheapest, but they have a lifetime warrranty on everything they sell, so when it breaks, they'll replace it no charge. I just swapped mine out yesterday at one of their stores, in fact.
I haven't tried the Spitjack or the F. Dick one. A little rich for my blood. But the Sur La Table one works well, doesn't leak, and held up for a couple years for me until the black plastic cracked at the base of the needle. I've broken every injector I've bought (Bayou Classic, the cheapie plastic ones at the supermarket, and a few others) but this one lasted a lot longer than the rest. I'm happy to know I can return it when this one eventually dies and they'll keep replacing it for free.