So after a probably unhealthy amount of watching Angel and Buffy episodes (there were about 250 episodes between the two shows), we finally finished both series.
Of the two, Buffy clearly remained past its prime. The last two seasons, in particular, were real departures from the initial snarky, angsty seasons. There's a fine line between angst and mopeyness, and and the last two seasons fell squarely into the mopey category. Television Without Pity helped for those less-than-excellent seasons, though.
After watching a couple of the first few Angel DVD commentaries it's amazing how much the show changed. Initially much more about life in the big city, it morphed away from the private-eye help-the-helpless motif into a couple major story arcs heavy on prophecies and alternate dimensions. And then finished in probably one the best series finales that I've seen.
All in all, the first four or so seasons of Buffy and all of Angel were pretty worthwhile. And if you haven't seen Firefly you really should. Buy or rent or borrow the DVDs, watch them in order, give it a chance, and you'll love it.
Oh, and Joss Whedon does the best commentaries. Interesting, self-mocking, funny. Good stuff.