Both the Samsung 840 and the 840 Pro are brand new drives that were just released a few days ago. Neither series has an established track record yet. As is my custom I always recommend waiting 6 months before purchasing a new drive to see if any problems develop.
A technical review of the 840 Pro ssd was published by AnandTech. During the review the ssd died. A second replacement 840 Pro also died. The cause of desth is not yet known. Here is a link to the review: