As a novel to showcase ideas, it succeeds. As a novel to showcase misogyny and thin characters in an attempt to bring real storytelling to hard SF, not so much. 
 I'll talk of the good parts first. I learned, or eventually recalled something that hadn't immediately made a connection to me right away but it should have. 
 The word Tau has a dual meaning in the text. One is Proper Time in Physics, and the other refers to coming full circle, both of which happens in the text. 
 Reducing Tau to Zero means they're going faster and faster and faster to the theoretical maximum of speed, even to the point of traveling between galaxies within weeks. They're already well and deep into the relative future by necessity of going so fast, making them realize that there's nothing left to lose because they've left everything they've known far behind. 
 The end idea pushes them outside of the framework of an oscillating universe and gives them the opportunity to pick and choose from the cream of any galactic honeypot along any time because they're outside of the framework. It's pretty damn cool, and even if the physics isn't accurate, the base concept that turned into the impetus of such an ambitious idea novel was striking and gorgeous. 
 It's both better and worse than Stephen Baxter's Ring, which, in hindsight, is an updated and expanded novel to do Tau Zero better than Tau Zero. The Ring had a lot more attention devoted to character, and although I can't say it was better, precisely, I can say that the development and progression into far time was a lot more fascinating, especially with the Human/Xeelee wars and the eventual grand-scale exodus from this universe. 
 Tau Zero was definitely a tighter novel, focusing on time and relative distances between stellar objects all the way to clusters of galaxies to the shifting of antimatter/matter oscillations underlying the fabric of reality. It was very fun to see Bussard Ramjets going far beyond their theoretical limitations, too, but I prefer Niven's treatments a LOT better. Hell, I was thinking during most of this read that I preferred Neutron Star. But by the end, Tau Zero pulled away from most of the similar SF pack by getting fantastical. (Sorry, I have a soft spot for Bigger, Kick-Ass, and Mind-Blowing concepts.) 
 Though, in the end, I agree with the Hugo awards for 1970. This was a runner up, and Niven's Ringworld won. Ringworld had Woo! You can't go wrong with Woo! 
 And that leads me to the not so great in this novel. It's not enough to ruin it for me, but I hated the treatment of women in it. It's not much different from SO many novels of the day, granted, but this crap really grates on me. It's like reading crappy sex scenes. My eyes kind of glaze over and skim till the meat of the story comes back. Tau Zero DID have story, too, an exploration of what it means to be cast adrift into deep time, losing their anchor to Earth and the possibility of ever meeting up with anything remotely like themselves ever again. It went through despair and a great deal of military psychology and a heavy reliance on democracy/committee-speak rather than a strict authoritarian rulership, which makes sense if you're trying to appeal to an American public, and some of the best parts of it were the attempts to keep morale up. 
 Unfortunately, the characters never did much for me. I'm SO SPOILED by modern SF and Fantasy. 
 That being said, it was still a great idea novel! 
 (So why am I reminding myself about The Number of the Beast by Heinlein? Because his ideas were even better along weirdly similar lines, and a lot more fantastical? Possibly. It's unfortunate that it also had some weird-ass hangups and sophomoric fixations, too. I'll never win! ;)