In particular, David Dixon is actually a better Ford Prefect than Geoffrey McGivern was. He confidently conveys a sense of alienness, and he makes the most of the rather boring scenes in the Vogon freighter at the end of the first episode. (Compare Fit the First of the radio series, where even his fans must admit that McGivern starts off sounding shrill and unsure.)
The other thing that works really well is, of course, the Book - the superb animations of the entries in the Hitch-Hiker's Guide, backed up by Peter Jones' narration - the one point where televising simply could not mean pointing a camera at actors reciting the lines in a stage setting. The combination of graphics and cameos - Douglas Adams himself stripping off and disappearing into the sea, the two unspeaking drinkers of the Pan-Galactic Gargle Blaster - are almost without exception brilliant.
There are some tremendously naff bits as well. Rather surprisingly, Simon Jones as Arthur Dent seems to spend a lot of time standing around as if he is waiting to be told what to do. Zaphod's extra head is simply embarrassing. The departures in script from the radio series (with perhaps the exceptions of the Dish of the Day, and the Disaster Area graphics, both of which had already featured in the novels) are not usually improvements.
One particularly weird bit of interaction is the chemistry between Trillian and Zaphod. In their first scene, when they hear the radio annoucner quote Eccentrica Gallumbits' description of Zaphod as "the best bang since the Big One", he and Trillian exchange what looks to me like a knowing smile. But then at the end of the fourth episode, when it looks like they are all going to be killed by the Magrathean computer banks exploding, Zaphod and Ford shake hands and sing a song, leaving Arthur and Trillian to look aghast and, in her case, very much alone. More could and should have been made of her character; she seems just a clothes-horse for skimpy red costumes. No big criticism of Sandra Dickinson intended - like Susan Sheridan in the radio series, she just isn't given much to work with.
Still, this was worth the (low) price I paid for it - especially the documentary clips on the second disc, which do add quite a lot.