As I am interested in applying my training for field sports, would braking (I don’t know what else to call it) be useful, i.e. sprinting for 10m for acceleration, then trying to stop dead or as close to it as possible to aid in direction changes, or is this a recipe for injury, as I already have a dodgy hamstring? 
xlr8, you describe in the other thread the wheel drill,
have you workied this into your agility training, we used to do a similar drill to this in high school…
there are so many different variations of agility drills out there (pro agility, nebraska agilty, etc) that we as fb player should look for the few that hit the most variables in changing direction, that is why im not crazy about pro agility, but we have to train it because of the combines dont we
Athlete,
In some sense, I consider much of the agility drill work to be remedial in nature. Think of it as similar to the use of A skips in sprinting. It is not the same as actually playing the sport, but it helps to reinforce good movement skills.
You should be getting a significant portion of your agility training during practices and playing the game.
So I think you are correct in that you need to find some drills that focus on the specific aspects you would like to reinforce and are comfortable with and then use them. They can be highly position dependent. For example, a running back does not need to work on his backpedal but this is likely to be critical for a cornerback.
I tend to like simple movement drills for my agility work and then put it all together during game-like situations. For example, the wheel drill works on one very focused aspect of your movement - decelleration. You obiviously need to use this skill in virtually every other agility drill, but when you add in more cones and more movement patterns, it becomes more difficult to focus on this one aspect. So break it down, then put it all together in a more specific manner.
Also, as you correctly point out, if you are tested on a specifc drill (like the pro agility) then it makes sense to work on it so that you can do you best - even if it isn’t necssarily the best drill for developing your football ability…