I've been using a Slap-Shot for the past couple of weeks and I love it. Particularly useful when dealing with a freaked-out sow trying to savage piglets and in need of Stresnil. I'm so happy with it, I would recommend to anyone. Cost around £20 with strong, reusable needles.
I bought a Masterject but returned it before the end of the trial period because I found it difficult to use. Might be okay out in the field, but you can't use it when you have to pen up pigs (e.g. for farrowing) because it's so unwieldy and obtrusive and they see it coming. You might be able to get it into the backside, but try to get it in the neck and they see it straightway because it's as big as a shotgun.