The success of the Buffy: Season Eight comic was followed quickly by the release of an official comic continuation for Angel, which I have also been reading. Although Joss Whedon does not write the comic, he has acknowledged it as canonical and continues to receive a plotting credit.
Overall, I would say that, while no comic version could ever entirely live up to the show, Angel: After the Fall has thus far fared better than Buffy: Season Eight. Part of this may be my lowered expectations after being underwhelmed by Buffy, but the idea of an Angel comic also just makes more sense. For one, unlike Buffy, the Angel TV series was canceled prematurely, and, while the unforgettable finale did not feel in any way compromised, there was always the sense that there was more story left to tell. Toward this end, the comic has remained consistent with the few details known about what would have been the sixth season of the show, specifically developments concerning Gunn and Illyria.
Apart from that, Angel is also a better fit for the comic book format. The structure of the Angel TV series was much more serialized than Buffy, with episodes often bleeding into one another and cliffhanger endings quite the norm. Compared to the outwardly inconspicuous cast of Buffy, the stars of Angel were larger than life, with more extreme, though no less charming, personalities that translate easily to an action-oriented comic. Even the shift from actors to artwork is less distracting. While there are no awesome covers as with Buffy, for the pencil work inside, the artists have mostly favored a more classically realistic style, focusing on dynamic action poses rather than facial expressions.
As for the dialogue, although writer Brian Lynch was not connected to the Angel TV series in any way, nor to Buffy, he has ably captured the distinctive speech of most of the characters, albeit they were more narrowly-defined to start with than the Buffy crew. Moreover, Lynch actually seems to be improving with each issue, such that I have no difficulty now attaching James Marsters's voice in my head to Spike's wry dialogue as written by Lynch.
It's not the TV series, no, but fans of the show looking for more are encouraged to give it a look.
I should probably also mention the Spike: After the Fall miniseries. As more of a supplement than a spin-off, it is also written by Lynch and retains the same feel and quality.