Yeah, Asterix in Belgium was the last one written by Goscinny. He died after writing it but before it was finished (it was written but not yet illustrated). Albert Uderzo didn't even want to finish it but was taken to court by his publisher and forced to do so. I actually like this story (it's quite funny), but you can definitely tell Uderzo was feeling sad while drawing it - cloudy skies, rain, etc.
Belgium was successful enough, and fans were vocal enough, that Uderzo decided to keep going. Udezo started adding magical and fantasy elements that were never part of the story, and in my opinion, never should have been. The magic potion turning people into stone (or shrinking them, or making them revert to youth), magic carpets, the bard (Cacofonix in English) causing rain when he sings, stuff like that (Uderzo actually touch on some strange magic in Great Divide as well, but it wasn't as prevalent). The worst was Falling Sky, which had Asterix and his village battling wits with a space alien that looked like something Disney would put out, with henchmen that looked like Arnold Schwartzenegger, and an evil alien that was supposed to look like manga. It was so bad that the "friendly" alien was named Toon and was from the planet "Tadsilweny", which is the letters of Walt Disney rearranged, and the evil alien was from a race called "Nagmas" (Mangas rearranged).
In that sense, the new writer is an improvement, returning Asterix to his roots. It just isn't as funny, though, and the most recent volumes seem to be only available in English Canada in the American translations, which means names of characters and places have changed (Getafix the druid is now Panoramix) and the words aren't nearly as clever. Traditionally, Canadians got the UK English translations .