I used the premade cartons for MONTHS after PFB was born. After spending weeks and weeks and weeks warming bottles to the perfect temperature while she was screaming for milk, I started bunging them in the microwave to warm up to room temperature. As others have said, the advice is not to use a microwave because of hot spots, but if you aren't heating it much then all you would get would be a slightly warmer spot. I decided to take the risk.
The great thing about the cartons is you can keep some handy with a sterilised bottle anywhere in the house, and feed them without even having to stand up. (I kept some on the coffee table in the living room).
A friend had the great idea of giving all bottles at room temp except the last one before bedtime, which they warmed up. I don't know if it made any difference, but a very easy way to introduce a bedtime routine with a warmer bottle!
I used Aptimel, widely available. I only chose this because it's what we were given in hospital and we decided to stick with it.
If you get your shopping delivered note that I often didn't get the full number of bottles from Tesco as it would have used up their inventory. So once you've decided on a brand that the baby is happy with, get a stock in so you aren't at risk of running out. I also once got Hungry Baby delivered by mistake, so worth checking.