The difference between a level camera and one that isn't straight is so extreme with a fisheye that you really need to use a tripod to take your time, get the camera level, check everything is completely aligned as you want it before taking the shot. The slightest angle puts everything out of line and distorted out of shape in every possible way! I never had to think so much with my other lenses before taking a shot. The Canon 15mm on a full frame camera has almost a 180 degree angle of view which is a whole new world if you are used to zoom lenses. You have to watch everything-particularly where your hand and arms are holding the camera and even your feet. It's very easy for them to stray into the edges of a shot without noticing.
The fisheye is potentially the worst lens for making people look beautiful but if carefully done, it could have a niche look you don't normally see. Hope to get some portraits with it soon... Keeping a flash out of the shot is going to be interesting!