This is such a hard question... It's hard, because most of us who breastfed our first babies remember that feeling of 'not enough', and our first reaction is to warn other moms against trying formula... believe it or not, all of this 'no, no, no!' doesn't come from a judgmental place - it just comes from experience and being unable to express our sentiments properly.
Most of us who tried supplementing had the same experience; the gradual slowing or stopping of our own natural milk production. I think almost all women mourn the end of our time to nurse our babies... we miss the closeness, and the ease. So we look back and try to figure out what happened... what we did wrong. Then we respond vehemently on message boards because we don't want other moms to make the same 'mistakes' that we did.
I was lucky, in that I was able to catch myself in time to fix the production slowing problem, and here's what I did: I took one day, every couple of weeks to spend nursing my baby whenever he indicated a need or want... I would stockpile snacks and cold water bottles (for me) in the morning around my bed, and we would just spend the day in bed nursing and napping and playing. I did this a few times (as much for the benefit of spending exclusive time with my baby as for the milk production) and I was able to nurse him exclusively until he was 14 months old... I only had to stop then, because I am pregnant and hormones made my milk dry up.
Whatever you decide to do, just choose to be at peace with it. Know that it's going to be hard (emotionally) after you wean your baby completely, and that beginning to supplement WILL lead to that place eventually. These are facts, not judgments.
Good luck!
-H