Although I am generally not a fan of study Bibles, I have found the ESV Study Bible to be a fantastic resource. I even bought one for my lovely wife, who has clung to her NIV Study Bible for many years.
One of the features that I have most enjoyed is that the entire contents of the ESV STudy Bible are available online to those who have purchased a print copy. In addition to all of the articles, notes, maps, diagrams, etc. you have the ability to store personalized notes and other neat features.
For a limited time (through March 31), Crossway is allowing even those who have not purchased a print copy access to the online site. You can find the details here. If you are at all curious about the ESV Study Bible, this is your chance to check it out for yourself. I do not think you will be disappointed.