Why would you recommend or not recommend That Was Then, This Is Now?
Some of the criticism of this novel is that it does not depicit teenagers in a very realistic way. Michael Malone, writing for The Nation in 1986, says, "The heightened language of her young narrators intensifies the glamour and sentiment of their stories, but it will not strike readers as everyday school-locker lingo." His main criticism is that the language is stilted and not very true to life.
However, Ryan Poquette finds the narrative compelling, despite some lingusitic set-backs. Poquette argues, "Hinton’s teenagers face adult situations, make adult decisions, and deal with adult consequences."
The question, therefore, is one of suspension of disbelief. Can you tolerate some perhaps not-exactly-true to life dialogue and get at the larger message within? That is for you to decide.
Jamie's answer is dead on. Hinton is often shown as creating teenage characters that appear as teenagers want to see themselves, not how they really are. Hinton's teenagers are all wise beyond their years with an uncanny sense of morality, mortality, and maturity. You don't have to spend much time in any high school classroom to know that her characters are very much dramatized. It would be tough to envision any Hinton character laughing at a fart joke.
However, I think it is that form of characterization that makes her novels so appealing to young adults. Teenagers so badly don't want to be who they really are; an escape into a Hinton novel provides them with a setting they can really be interested in. I would absolutely recommend any Hinton writing to any teenager for this reason.