I've used Amber before in a Porter. It's similar (to me) to Biscuit or Victory, except more intense, a little redder, and slightly roasty. I think @ 5% it won't be overbearing, but I wouldn't use a large % in a pale beer. It is really good in Brown, Porter, or Stout though.
This is a really good description. I've been experimenting with some different "character" malts in my APA, and used 5% Amber in it recently. I really like the result, but that little bit of roasty edge probably pushes it out of an APA if you're worried about style guidelines. I think it would be great in a bitter, amber, brown, porter, etc.