I loved this series a lot.
If you haven't seen either Buffy or Angel yet, the best thing to do is buy them both, then see them in this order:
Buffy seasons 1..3
Angel seasons 1..4 and Buffy 4..7
Angel season 5
The series (that is, Buffy + Angel) is extremely addictive. There are also a lot of books out there, some of them quite good (especially those written by Christopher Golden or Nancy Holder). There's also a "Buffy season 8" which in fact is a set of 5 graphics novels (so far), not a DVD box.