I've just reread the first four bukes and the last two are nowhere near as bad as I remember them. Mind you... I guess it's the Catcher in the Rye syndrome: you just have to accept that HHGTG was a one off and things would, in the name of my ex, never be the same again.
LUE is a decent slab of sci fi in its own right, but the flaw is that it wasn't a HH tale but a Dr Who one, so he had to force the humour in instead of letting it arise naturally. It also explains why the heroes' actions are so out of character: bumbling fumbling Slarty becomes an authority figure, Ford decides saving the universe is more important than partying, and timid nerdy Arthur swoops down to lop the head off a killer robot. Hmmmm....
SLATFATF is still the weakest, and the presentation of women still makes me cringe, as does the editorial comment about 'skipping to a good bit with Marvin in'. Again, cutely whimsical in itself, but not really HH.