Most Bibles are fairly trustworthy. But translation is an art. Even in English, one word can mean so many things. So a translator chooses a word in his language which he thinks suits the word best. For instance the word in Hebrew, the word for bless and the word for curse are the same things. So Job 2:9 can be translated, "curse God and die" or "bless God and die."http://bible.cc/job/2-9.ht
The world's knowledge of history and language has grown over the centuries. The KJV translators didn't know as much as modern linguists do about Greek Vocabulary, grammar, and punctuation so they miss a lot. Look at how the different translations translate Mark 9:23 http://bible.cc/mark/9-23.
The KJV is honest though. When they guess at a word they put the word in italics. Or when they added a word to make the sentence "clearer", they put that in italics also. One of many examples is Romans 11:4 The translators slipped in "image" of Baal. Either because they didn't understand the word there or because there was no word there and they want to make it clearer. It's always best to read the KJV verses with and without the italics. Read the verse without the italics, then with it. For instance the KJV writes "a coat of many colors" and modern translations write "an embroidered coat with a long sleeve." I like the KJV translator's honesty. Most modern translators don't tell you where they added some words. And usually (not always) the KJV additions are right.
The KJV is fairly trustworthy, although there was spiritual battles between denominations and the translators compromised on what words to use to please one side or the other. And it depends on who was translating a particular book. For instance in Acts 28:9, the KJV writes, "others also who came were healed." But in other translations, the verse reads, " the rest of the people on the island were healed." It's subtle but it's as if the KJV translator of ACTS couldn't quite bring himself to believe that everyone was healed and simply said "others also."
Sometimes a translation is right but our English minds read it wrongly because of English words. For instance, Paul writes "see what big letters I write" and because the word for "letter" in English can mean an actual letter or a message, you have people thinking Paul had eye trouble. Another example in Hebrews 5:14 where it's translated word of knowledge your "senses trained" and the meaning is that we have our five senses are trained to discern between good and evil: our sense of smell, eyes, ears, taste, touch, can be trained to discern and have a word of knowledge. Plus so many English words have changed meaning since the old KJV days. "The darkness comprehended it not" means "the darkness did not overcome it" but so many people read the KJV and think the verse is about the darkness not understanding.
My favorite Bible translation is Young's Literal translation and also the KJV but I also have an Eastern Text and a French and a Spanish Bible. One can never have too many Bibles. I always go to bible.cc to look at all the verses.