3some help- please share your experience!

Let me preface this by saying that 3some naysayers need not apply here! I'm not looking to hear about how against it you are or how bad you think it is to do...I'm only looking for open minds and real life experience please!
Now to the point: my hubby and I have been married for 3.5 years but together for almost 11 now. So the other day we were talking and got on the subject of 3somes and expressed some mutual interest. Before we really decide to go down this road and set our rules I want to hear about some real experiences.
-What rules did you and your SO have?
-what were some positive pieces of it?
-what were the bad parts of it?
-do you regret it?
-how did you even go about entering the experience?!