What has his performance been like?
Andrelton Simmons is known for his spectacular defense from the shortstop position. He's also hit for a solid batting average every stop in the minors and even last season he hit .289 in his 49 games in the majors. Simmons' solid contact rate, decent speed, and plate discipline all say his batting average will usually be above league average. He's got a solid approach at the plate, limiting his strikeouts while taking his fair share of walks.
What is his current status?
The fact that Simmons has been in the leadoff spot for the Braves most days is a great sign for his future value. The Braves have a tremendous lineup and this will mean that Simmons should score plenty of runs. Simmons is only hitting .233 thus far this season which can be easily explained by his .239 BABIP. With neutral luck he would be hitting a lot closer to his .289 average that he had last year. He also has 11 BB with only 12 K. Simmons has 2 HR and only 1 SB thus far but it's likely that with a little more experience and his impending batting average improvement he will end up with 10+ SB this year.
What can we expect moving forward?
An average in the .280-.295 range with at least 8 HR and 10 SB is nothing to sneeze at from a 23 year old shortstop. That is what Simmons likely ends up with at the end of the season. Eventually his SB total could end up at around 20 a year. He had 10 SB in 44 games at AA last year while being caught twice. I wouldn't expect his HR power to develop much but that's okay. The time is right now to buy low on Simmons, especially in keeper or dynasty formats. He's only going to get better this season and he's only going to get better moving forward in his career development. If he manages to continue to hit at or near the top of the order in this Braves lineup as his batting average rises and he gets more opportunities to steal bases then he will eventually be a sure thing starting SS, even for 10-team leagues. Let his batting average scare others away while you know better.
Here is the piece from December on Simmons: http://www.faketeams.com/2012/12/6/3724612/andrelton-simmons-have-glove-will-be-relevant
The biggest difference might be his perceived value. Whereas he was a popular sleeper SS during drafts he is likely frustrating many of his owners with his performance thus far, creating an opportunity for anyone who doesn't currently own him.