I pumped the majority of my LO's feeds and gave myself terrible mastitis; I ended up in hospital on a drip for 2 days
When pumping all of your feeds, you end up totally emptying your breasts everytime you express, they then fill very quickly which can be very sore and uncomfortable. This is because when you breastfeed, your baby takes as much as they need per feed, then your boobs fill up again according to how full/empty they are after the feed.
In order to express all your feeds, you'll need to use the pump every 3-4 hours, which with a newborn is really disruptive as well as exhausting (just think when your LO goes off to sleep, you normally have to jump on the pump for an hour to express their next couple of feeds when really you should be getting your head down as well...!)
From experience I found that because I was expressing all the feeds (and getting 3-4oz a time per boob) I had to express at least 3 or 4 times a day or I would leak through my bras onto my top - my boobs were literally overflowing! And you can't not express as it is simply too uncomfortable.
I would say if you do want to express to share the feeds with your partner, get a decent electric pump; the Medela Swing is the best one, to wait until your LO is properly latched on so as not to confuse him/her with nipple vs teat, wait for your milk to come in (normally around 3-4 days) and that you express small amounts at a time, no more than 2oz. I loved the idea of expressing as it seems you can feed the baby from a bottle and know exactly how much they're having but I
really overdid it and got really poorly. I would advise against it to be honest, but that's just from first hand experience.