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 death is not yet known. Here is a link to the review:http://www.anandtech.com/show/6328/samsung-ssd-840-pro-256gb-review
Other technical reviews were very favorable.
In the meantime I recommend the Samsung 830 ssd. It has a stellar track record that is hard to beat. The popular Cucial m4 is another ssd worth considering.
I maintain the ssd database listed in the sticky at the very top of this ssd forum section. Here is the link:http://www.johnnylucky.org/data-storage/ssd-database.html