Buy if:
1) you have the money
2) you need the flexibility of being able to watch them when you want to watch them
3) you will watch them over and over again
4) you want the sense of ownership
5) add other factors here
Otherwise, borrow them from the library. A good strategy would be to borrow first and see if:
1) you really have time to watch them
2) you really enjoy watching them
3) you really want to buy now that you have already watched them
I personally buy (but don't watch very often due to lack of time). I don't mean I have a lot of money to spend. It's just that here in HK, we can buy the China editions of DVDs, which are very cheap (US$3/DVD). It is completely legal and the content is just the same. The HK editions are like US$15-20, far more expensive. I have already collected 64 titles so far. Nearly all are piano related. I have been trying to find "Horowitz - the Last Romantic" without success. I am prepared to pay quite a bit if I can find it .....