Fantasyfootballcalculator.com has Martin's average draft position at 25th overall, meaning he's being taken at the top of the third round in most drafts.
His value is constantly ascending, though, thanks mostly to his excellent preseason and training camp. He should be viewed as a mid-to-late second-round pick in most formats, with extra value in points per reception leagues.
Martin currently has no injury concerns of note, meaning you can feel comfortable taking him early.
Making Martin an even better pick is the fact that he has taken hold of the starting situation from LeGarrette Blount. ESPN projects Martin to receive around 60 percent of the carries, which sounds about right. Martin should receive significantly more reps in passing downs than Blount, upgrading his value in PPR leagues.
Fred Jackson and Trent Richardson are two similar-tier players that you should target ahead of Martin. Jackson has a chance to make a big-time rebound this year, while Richardson has more upside if his health doesn't remain an issue. Both should receive more carries than Martin this season.
Martin is a better pick, however, than many similar-tier players, including Maurice Jones-Drew, Ryan Mathews and Adrian Peterson. Mathews and Peterson have major injury concerns, while there are no guarantees that Jones-Drew will play for most of the season thanks to his contract dispute. Martin is simply a safer pick than these guys.
You can expect a solid season from Martin, with numbers in the neighborhood of 200 carries for 850 yards, along with around 45 catches for 400 yards. Martin won't be a huge touchdown producer, but he should provide you with a few long touchdowns. Between five and eight touchdowns is a reasonable expectation for the young back.
All in all, Martin's good health and explosive potential makes him a worthy second-round pick, but the presence of LeGarrette Blount limits his upside for this season.