As I once joked, my favorite season of Buffy the Vampire Slayer is seasons one to three. The show did change after that, but I don't think it was solely the loss of Angel and Cordelia (though it certainly had an impact.) A number of things impacted how the show would be afterwards—the loss of the school environment (college just isn't the same), Principal Snyder, as much presence of Joyce, Buffy's more hopeful nature, Willow and Xander's closeness (not gone but definitely changed), etc.
I like Angel a lot, but his leaving didn't just seem like a gimmick to get him on his own show. It felt like a natural point in his arc as a character and in their relationship. It seemed right when it happened—at least to me. I wouldn't have been upset to see more of him, but I think it would have reached a point of diminishing returns quite soon if things hadn't changed drastically.
I guess I don't think it made the show better or worse, but I think it was the right choice to make, especially for Angel, who got to come into his own on his show, not only in terms of character development, but in terms of what he was allowed to do (for instance, note how many fights he's allowed to win on Buffy versus his own show.)