I have done. I have stayed at home with both of my children so I never needed to express milk in large quantities to leave for them while I was at work, but I have expressed a little to put in a bottle when they were staying over at their grandparents' house (not when they were really little and having only milk, when they were over 6 months and on solids). I found it pumping milk very time consuming and tedious. I can't get out anywhere near as much milk that way as my kids can drinking directly so it takes me about three sessions of pumping to get one feed's worth. When people make dumb comments about how breastfeeding mothers should "just" express milk and take it in a bottle when they're out with their child I suppose they don't realise how difficult it actually is for some women. In asking me to do that you are asking me to waste up to two hours of my time (not to mention the hassle of bringing a bottle and keeping it at the right temperature) just to make you feel more comfortable. No thanks. Breast pumps can be useful, but just because they exist doesn't mean you have to use them if it's not convenient for you or you just don't want to.
Also, there are pumps and pumps. The first manual pump I tried was rubbish and more milk leaked out than went into the bottle. The next one (an Avent brand one) was much better.
My medela breast pump was an absolute saviour for when i wanted to expel milk when my breasts were engorged for the first month after having baby, also it was so handy when wanted to pop out for an hour or two and leave the baby with a relative.
I had a breast pump with my first baby - I got a fancy medela one after the crappy one I was handed down just didn't cut it. My first was never able to latch on, and I tried for 6 weeks to get her to, expressing as much as I could, which took hours and I would just sit and watch tv while trying to express, and then more often than not she just threw it up anyway, so after 6 weeks I gave up and put her onto formula. My next two fed straight away, and with them I used the pump occasionally for babysitters so I could go out, but because of the time it took I didn't use it too much.
Some people have to if they are breastfeeding and going back to work, or if for some reason their baby can't latch. Some also donate excess milk to other babies who need it (eg. if their mums are sick and receiving medication that means they can't breastfeed).
Yes, you can keep the milk in the fridge for a couple of days or the freezer for a month (I think, I'd have to check, it's been a while). It's not as good as fresh but still better than formula (which is in turn much better than nothing, before any defensive formula feeders jump on my comment).
I expressed manually for a while before buying a manual breast pump. Though it was handy, I would buy an electric pump if I could go back. Actually, I don't know if I would buy one at all because Baby soon decided he didn't like the bottle, so I don't express much anymore. This can be inconvenient though, as I am limited in the time I can spend away from him for work and the like.