Tau Zero exemplifies much that came out of the pulp SF tradition. On the credit side, the idea is brilliant, and it's eloquently described in the Amazon "Product Description". The novel is probably SF's most thorough attempt, at least at time of publication, to explore the implications of relativity for travellers on a starship that's approaching light speed.
The trouble is, those implications are so far-reaching they don't really leave any room for a plot - they ARE the plot. Anderson correctly realised he needed some strong human interest to make this into a novel. Unfortunately, he wasn't up to the task of providing it. The weak characterisation, which comes as standard with a lot of SF, is more of a problem than usual because Anderson is trying so hard to avoid it, but failing so badly. The attempts at characterisation mainly come from dialogue rather than action, and said dialogue is among the most excruciatingly implausible you'll ever encounter (I kept hearing Tony Curtis, in Some Like It Hot, imploring "No-one talks like that!" as I read it). The cast speak in psychobabble paragraphs rather than demotic conversational language. And they all sound the same, so it's really hard to tell who's who. The only character who stands out at all is the hero, the ship's security chief Charles Reymont. Unfortunately, he's a Randian superman, or, in plain English, a complete eejit, and he causes irritation after irritation as he goes through his obligatory duties of demolishing straw man arguments and giving some sweet space lovin' to the women on board, all of whom, as nothing more than wish-fulfilment figures, can't resist whatever it is he's packing in his spacesuit.
Tau Zero is highly regarded by a lot of SF fans, possibly because it reasserted "traditional" SF virtues at the height of the New Wave era, possibly because it at least tries to have some characterisation and human interest, and possibly because Anderson was so widely liked in the SF world. To be fair, while he was never a great writer, he was far from negligible, and he wrote much that is better than this.
I read to the end with increasing annoyance but stuck with it because I was genuinely interested in what happened to the ship. I didn't care about the people on it though. Which is odd, because I'm not normally a huge fan of hard SF. So: five stars for the idea, one star for the characterisation, three stars as the halfway point between the two.