I exclusively express for my 3 month old. (well she does get the odd bottle of formula)
We do this because she had latching problems, and whilst I would recommended this to someone in the same position as me, I am not sure I would recommended it otherwise.
It is hard finding the time to express, and can double my workload having to wash and sterilise bottles and pumps, having to pump and all this before she even gets fed.
Once you are in a routine, it gets easier, and I invested in a double pump which made my life so much easier. I also love that my husband gets involved.
I suppose the biggest downside is that I can see my supply dropping, so I won't be able to bf for as long as I wanted to (am currently aiming for 6 months)
Good luck with what you decide, and if I can answer any questions, I will try and help!