I'm feeding 6 month old DS on (mostly) expressed milk.
DS was born 6 weeks early, and tube fed for the first 2 - 3 weeks, so expressing milk was initially the only way to get breastmilk into him.
We sadly couldn't manage to establish breastfeeding, but as I'd got into an expressing routine I kept that up.
As others have said, it can be hard work, mostly in terms of finding the time to express and sterilise all the equipment, especially if your LO wants / needs lots of attention when awake. And at first, to establish the milk supply, you'll have to express 8 - 10 times a day, not letting more than 4 hours go by between times.
If you're planning on exclusively expressing, you will need a good reliable electric pump - I express from both breasts at the same time to speed things up, and DH bought me an 'expressing bra' off the interweb so I can do things with my hands at the same time. Although an old bra with holes cut in it would work just as well.
It's also harder if you want to go out and about with baby - you can't practically express when out, so that cuts down the number of times you can express, which can reduce milk supply.
That's the main reason DS isn't fed exclusively on expressed milk for us - if I express every time he has a bottle, I can keep up with him, but if I miss expressing because we're out at a baby group or friends house, we can end up having to fall back on formula.