The fourth entry in the Hitchhiker's series is significantly different from the first three. Almost none of it takes place in space, it's missing major characters from the series like Zaphod and Trillian, and characters like Ford and Marvin have very little appearance time. The book is almost solely about Arthur Dent. Not that a story about Arthur Dent is a bad thing, indeed the story has always revolved around him in a way, but in this book it's less of a science fiction adventure comedy starring Arthur and his wacky buds and more of a romantic comedy starring Arthur and his new love interest.
So yes, it is true that So Long, and Thanks for All the Fish takes place almost entirely on Earth, is missing most of the usual cast, and is in fact a love story, but, though it may sound as if it is the farthest thing from a Hitchhiker's story, it still maintains much of the charm from the previous entries in the series.
For one thing the absurd and witty humor is still present though it is admittedly less bizarre and satirical (it would have been nice, I think, if this book was more satirical of romance, similar to how the first book was highly satirical of sci-fi stories) compared to the first three books. There's still plenty of surreal and wacky situations that the characters find themselves in, though it is, once again, admittedly much less bizarre and insane compared to the first three books. The novel does have its moments of greatness (see Arthur's biscuit story), and is worth reading for that fact alone, it's just a bit more tame.
To describe this book I could do no better than saying that it is all the greatness of the first three novels, but more toned down. Which I suppose is a bit disappointing, and it undoubtedly fails to reach the level of the first three, but it is still a fine book and well worth reading. In fact I see it as a testament to Douglas Adams' writing abilities that even when he's not writing about insane adventures and absurdities his writing alone can still carry a story and make it highly enjoyable (I again must cite the excellent story Arthur tells about the biscuits as evidence of fine writing).
A three might be a tad harsh (though still a pretty good rating) but I do feel a four would be a bit too generous. It was a highly enjoyable read--I read it in one sitting--it just comes off as much less impressive when rivaled against the standard that the previous books set. Let's call it a 3 and a half out of 5.