These actually came out some time ago now, and yes, a few people have read them. Mostly serious UFO researchers, I suspect (as you’ve spotted, they’re hardly light reading), but also a few journalists looking to glean titbits for a silly-season story – “Britain’s X-Files Revealed!” etc.
As to whether anything ever comes of the investigations: very occasionally, yes, but possibly not what you’d expect. DI55 only investigates cases that seem like they might have some national defence implication: they’re not particularly in the business of explaining UFOs (or Unidentified Aerial Phenomena as they call them these days) per se, they just want to make sure it isn’t some new Russian secret weapon or similar. Once they decide there’s no threat, they close the file.
Occasionally, though, they have learned things of significance from UFO reports – I believe there was one case where a number of people reported lights in the sky which turned out to be a Soviet rocket booster burning up on reentering the atmosphere, helping to confirm the launch of a spy satellite.