This is "based on Douglas Adams' book" well, the names are the same. Not a huge problem though as I think Adams would be happy with this programme.
Dirk Gently runs a "holistic detective agency" and believes in the inter-connectivity of events. He is accompanied in his quests by Richard MacDuff. So yeah, it's Holmes and Watson to some extent.
What I like about these programmes is that they don't dwell too long on analysing the crimes or digging into the characters' backgrounds. The writers concentrate more on providing a slightly off-beat story. While features of Dirk Gently (and to some extent MacDuff) are constants through all 3 programmes, there is no attempt to shove them "in yer face". There is also a degree of ambiguity about the setting although I guess it is around the time it was produced. I sometimes think producers place too much time on getting the background right while, quite frankly, I don't care.
Stephen Mangan is a tad irritating in the first programme with his Holmesian deliveries but this begins to work the longer you watch. He carries the lead role well. And Darren Boyd has never impressed me in the past but he works well as the sidekick. I like the portrayal of the Janice/secretary character. It is quite one dimensional (not a bad thing) and isn't over used.
The producers have struck a balance with the actors, story and setting. I am disappointed that this looks like the only time we will see this combination as I enjoyed all three episodes.