I can totally see where the negative reviews come from, it IS a rather sanitized version of Bloom County/Outland. But that being said it's still a fun little Christmas special that's suitable for all (Ok, most) of the family.
The premise and plot are pretty formulaic with Opus desperately desiring "wings that work" and appealing to the one authority he knows that can possibly help him. Santa Clause. And most of the episode is pretty much his attempts to overcome/cope with his flightless status until Santa can get there.
The jokes are pretty good, even today, and Bill The Cat naturally steals the show.
I think what pisses people off about this is that it's missing the raunchiness that they were familiar with in the comics, instead favoring the family-friendly storyline.
Personally I still think it's worth a watch whether you're a fan of Berkeley's work or not.