This is a question I get asked almost everyday. The answer is yes, and no. The information shown on Credit Karma, such as which accounts are reporting, payment history, balances, collection accounts are completely accurate.
However, the credit score given on Credit Karma is not always accurate. Many free sites offer a VantageScore 3.0, rather than the industry standard FICO score. The VantageScore 3.0 is on the same scale as the FICO score, 300-850. The problem is that the algorithms used to calculate the FICO score are proprietary, meaning they are top-secret, and FICO is very secretive about the exact calculations. The VantageScore 3.0 is a knockoff of the FICO score, but it is not always the same or reliable. On average the VantageScore is 26 points lower than the FICO score. Overall, Credit Karma is a probably the best free credit-monitoring website, but take the score with a grain of salt.