Clearly, Douglas Adams is a genius. But actually, I don't like these very much. I found them much easier reading than when I first read them as (relatively speaking) a child, and I thought that, for this reason, I would like them more than I did then. But no.
Part of the problem must be that almost nothing could live up to the Hitchhiker's Guide, but even considering that, the Dirk Gently novels just don't seem all that good to me. They are funny in places, and made me laugh out loud a couple of times. But they are flawed. Both of them seemed to me to suffer from being hurried (I've heard that Douglas Adams often wrote in a hurry, and, according to rumour, after his publishing deadlines, but I don't think it normally showed).
Dirk Gently's Holistic Detective Agency is apparently a re-work of an unfilmed Doctor Who script that Adams wrote in the eighties. That is probably why its story seems more intricate and coherent than The Long Dark Teatime of the Soul, but it doesn't appear to have changed much in the re-work. We have Dirk/Doctor, a bizarre, larger than life character who is prepared to look beyond the ordinary, who is able to see further than others, and we have clients/companions, swept along in his wake, playing the bumbling Watsons to Dirk's Holmes (well, the usual stereotype of Watson and Holmes, anyway). And we have a mystery beyond the ken of any of the characters but the hero. And actually, that is where my problem is, I think. In both of the Dirk Gently novels, the central mysteries defy any kind of logic at all.
I have the same problem with the TV series 'Medium', which has a US District Attorney's assistant able to solve crimes by psychic powers, but unable to share her secret with anybody outside her immediate friends and family. The problem is that this is internally inconsistent. If she had the powers, and was able to solve crimes in the way depicted, there would be no problem telling the world about it, because she would simply be able to prove herself right. Dirk Gently is a little like that, in that the whole universe has to be bent around the character for him to be effective. The solution to the mystery in the first novel relies on (some of these probably count as a spoilers...) the existence of ghosts, the possibility of time travel, the invention of a time machine on Earth hundreds of years in the past, the existence of aliens able to cross interstellar space to reach Earth yet unable to prevent all of their deaths at the hands of a simple mistake, the ability of the ghost of one of those aliens to possess the mind of humans, and on the design of a machine on another entirely alien world sufficiently resembling a human that it can pass off as one without comment. Its a huge stack of ridiculous improbabilities that needs the Heart of Gold back in order to make sense of it all.
To get to the point; my problem with it all* is the whodunnit thing: there is nothing clever about having your character find the solution to a crime/mystery that you yourself have invented. Whodunnits must be told very well to draw you in and preserve the illusion that the world they inhabit is not the creation of an author who can pass the answers to the central character. Which is where (for me) the Dirk Gently novels failed. It's all too much. There is a lot of entertaining writing in there, but as a whole, the novels don't really work.
Incidentally, The Long Dark Teatime of the Soul suffers from another insurmountable problem: a novel called 'American Gods', by Neil Gaiman. Same premise, handled slightly differently...
* Other opinions are available.