This play is brilliant and I'm sure it deserves 5 stars. My only reason for knocking it down half a star is that you really can't read it in a vacuum. You have to have some knowledge of Hamlet for it to work.
Rosencrantz and Guildenstern are Hamlet's two childhood friends. This play is written about the time that they are off-stage in the play. Not the time the actors playing these parts are off-stage but the times that the characters are off-stage. Essentially, at those times they have no purpose because the playwright hasn't given them anything to do. This is their search to figure out who they are when there is no script. It's like looking at the back of a piece of tapestry. What matters is what's on the front and everything on the back is just a jumble of seeming meaninglessness.
Rosencrantz and Guildenstern aren't even sure who they are when they have no scripted parts. When their parts are on, they speak in brilliant Elizabethan English. When they are off, they struggle to figure out what's going on in the world and the dialogue can get confusing, deliberately so. It's very funny and quite clever. It deals with issues of reality and identity. I don't usually enjoy modernist literature but this is one of my favorite British dramas I've ever read.
If you were to read this cold, with no background, you'd probably give it up like I did years ago when I first had the book. I couldn't make heads or tails of it but with the right bit of knowledge going into it, it's just really fun and thought provoking. Tom Stoppard himself put out a movie version that's available on Youtube with Gary Oldman as Rosencrantz. It doesn't follow the play in every detail but it is great fun to watch.