I am not a Tombstone expert, but I have visited several times, talked to residents and have read several excellent books on the subject, including one called, "...and Die in the West," by Paula Mitchell Marks." This book is an excellent accounting of the Earp era. Paula actually researched her subject and ready the accounts of the day, including the Tombstone Epitaph and the court records of the trials mentioned.
The movie accurately portrays several events I have read about in my personal research, and therefore believe to be true. The movie even includes actual dialog from the day. While it is still a movie, and is not a 100% truthful/accurate portrayal, it is pretty close to portraying the actual events of the day, much more so that the other movie of the period, Wyatt Earp, starring Val Kilmer (still a good movie.) But for truthfulness and accuracy, This is a very good movie, definitely worth watching if you want to know how it "really" was.