I usually prefer "Angel" over "Buffy" - darker, complex characters; a minimum of quirky Scooby-speak; and it's more violent, definitely more violent.
David Boreanaz carries the show well. His character is more interesting when he's darker. Unfortunately, he wouldn't have a supporting cast if he remained that way. Charisma Carpenter, who played a relatively two-dimensional character for 3 years on "Buffy", really comes into her own on this show, adding depth to Cordelia and proving that she can act rings around Sarah Michelle Gellar. Alexis Denisof has done a good job of making me forget that Wesley was an uptight, stiff-upper-lip comedic relief in the previous series, and turned him into a more likeable, stronger character - almost a younger version of Giles. Recently, the writers have been ignoring Gunn in favor of regular guest stars Lindsey and Darla. Now maybe that particular story arc is over, they'll give J August Richards more to do.
This series is strong and deserves to stand on it's own merits, not simply as a "spin-off".