I am tempted to get one installed for my 5 month working cocker pup. She likes to go out ino the garden, then back in, then wants to go out again, and I am not sure it's good for the human/dog psychology (where I am trying hard to be the leader, not her!) to be at her call whenever she chooses! She doesn't need the loo this often, sometimes she just stands at the back door since I have opened it and doesn't even go into the garden - and I'm sure she thinks I am a fool for doing it!... 
Anyway - my main concern is a small but VERY vocal dog next door. She's a lovely dog - but outdoors most of the day and I guess likes to protect her owner's property. I haven't got problem with her (next door) barking, but would rather prefer my nice, quiet dog stayed that way, and not to become like the loud dog...
Do you think she'll end up adopting the other dog's behaviour if she spends as much time as she fancies outdoors (courtesy of dog flap) when I am out of the house for a few hours or so?... At the moment my dog doesn't spend much time outside alone, she'd rather be with me indoors.
Just wondered if you have any advice/experience on this.
Thanks everyone :)