We will start with the reasons to sew your own bras:
1. Other sewers are doing it and the results are impressive. No one has said it was too hard to do. There are some great photos on the now closed Pattern Review Lingerie contest.
2. This is one area where you can actually save money sewing and there aren't many left. In a moment of trance-like madness I got a proper bra-fitting last year (worth every penny because I have been two band and cup sized wrong my whole life) and walked out with a $250 bra. Am I insane? Apparently that day yes. Well this bra is now the only one I have that really fits and I'm not paying that again, obviously. Cheaper right size bras haven't fitted well at all and they are just too structured with too much gear for me.
3. You can get exactly what you want. I often wish I had a supportive cotton stretch bra that had support but didn't look like a runners bra. I don't run except to the fabric sales.
4. These are small things and there is the possibility to get creative and make matching sets with scraps of fabric.
5. You might get more comfort.
Reasons not to sew bras:
1. As Robin points out you don't know if this thing is going to fit until you are done. So by definition these projects have the potential for considerable sewer's rage, throwing wadders across the room, doubting yourself as a capable seamstress, and buyer's remorse over all that cash you spent on supplies for such a Stupid Idea. (I didn't actually experience this, much, but could visualize it as I understand my sewing personality).
2. It would look homemade, like something that got put together in the Depression from feedsacks.
3. Come on you can't make everything and sewers every where need to be careful of exposing themselves too often to those moments where you actually go inside a store and find that the thing you spent so much time and money perfecting with online fabric purchases, plus shipping, 400 hours on online pre-sewing education to fine-tune fit and technique, and 4 different patterns, is on sale in the discount store in exactly the right colour, size and fabric for $15.