I just wanted to share my experience. After a ped visit when my baby was 1 week old, I decided to give myself permission to give him a single bottle of formula up to once per day. I think that for me, this choice was an absolute lifesaver and a sanity saver. And since I was not experiencing any problems with milk supply, the choice was purely about giving myself a break once per day from breastfeeding.
As a single mom, with no car living in a small town and no help who was stuck inside with a newborn during a very cold winter, I felt really overwhelmed with having to be responsible for every single one of my baby's needs. Perhaps if I had a partner who could have cooked or shopped or done laundry once in awhile or changed a diaper or done ANYTHING at all, breastfeeding 10-14 times per day would not have felt quite as overwhelming. But being single and responsible for EVERYTHING left me needing to find some small space or time for myself. Obviously, I couldn't take a break by actually leaving the baby, but a bottle was a small space.
My son ate every 40 minutes to 2 hours for the first couple of months of his life. Giving him a bottle every once in awhile, meant not just a break in terms of time, but more importantly, once in awhile it gave me a stretch of time in which my body was my own again. My son nurses and co-sleeps and I'm very happy with those choices but I really think that it is likely if I had not allowed myself to occassionally supplement, I would have either given up on breastfeeding or given up on co-sleeping. I needed a tiny bit of freedom in those first days and using a bottle a few times per week gave me that sense of freedom and that I had a self outside the baby.
As it was, we got through those first two months, and I have (almost) never used formula again. Since the time that he was 10 weeks old, I've been exclusively breastfeeding on the 4 days per week that he is home with me and he gets almost exclusively EBM on the 3 days that he goes to day care. Each day, I send him with 3 bottles of breastmilk and one of formula. He drinks the three of breastmilk --once in a great while, he has very hungry day and also has a fourth bottle (formula). But 9 out of 10 times, he does not get to the fourth bottle and given how difficult, I find pumping, I decided that I'd rather have a bottle of formula thrown out almost every day than a precious bottle of EBM.
Come to think of it, although I gave myself permission to give him formula once per day, I ended up actually giving him formula far less often than my "permission" allowed so it was more about giving up the guilt than about actually using the formula. Prior to the ped visit, I was so worried about nipple confusion or lessening my milk supply or about early weaning, that I was determined not to give him a bottle for at least a couple of months and never formula. But honestly, I think in my case, my initial refusal to use a bottle just caused me stress and guilt.
The truth is that the occassional bottle (it ended up being a few times per week), never interfered with my milk supply, I still have plenty for him. And at six months, he clearly prefers nursing to drinking from a bottle and certainly prefers breastmilk to formula. My hope is that this will remain the case for at least a year or more.
By the time that I went back to work when he was 10 weeks or so old, life had gotten a little bit easier and I was less overwhelmed. Plus, I had 3 days per week at my job which gave me a break of sorts (and some adult company!). Thus, I no longer felt the need for a break from breastfeeding so I rarely use a bottle now and the rare times that I do, it is always thawed breastmilk.
I know that someone wrote that if the woman needed a break, she should use EBM. That might work for some women, but I find the whole cleaning the equipment, pumping etc process extremely onerous and I never produce much milk. It would not have been a break for me to give up a nursing session in favor of a pumping session. A break was pouring a can of liquid formula into a disposable liner so that all I had to do was feed the baby and then wash the nipple and collar later. I did not even use the powder mix. So while I did not do it often, it was a huge help when I did --often when I was feeling absolutely at the end of my rope.
In fact, I could not even find a spare moment to research and purchase a good pump for my needs (in my price range) for more than a month after the baby was born. Figuring out which pump to buy and learning how to use it was a difficult process and there was no way that I would have been able to use EBM in the first few weeks after he was born. In fact, I still can't pump and nurse at the same time, I just don't seem to be that coordinated. I really can only pump when the baby is at day care and I did not have child care in those first two months.
my 2 cents