I was going to ask the same question. Rotoworld likes them a lot. I have the Carolina defense (facing KC this week), so I explored another option for this week...picked up JAX from the wire. I may just keep them for the rest of the year. There were better matchups for this week, but JAX is going to be solid for the rest of the year.
Like said, their run defense is very stout, led by the two maulers up the middle...Stroud and Henderson. They won't give up many points at all. Fast LBs in Peterson and Ayodele and a good, solid secondary with Darius, Grant, and Rashean Mathis, a very athletic corner whom I liked a lot coming into the draft last year.
Here's the problem. They have a very lacking pass rush (not many sacks) and do not force many interceptions, either. They will force some fumbles, but that's it. That will hurt you in most fantasy formats, since popular ones award high points for sacks, int's, etc., and do not value points given up as much. In addition, this unit won't give up many yards and that is completely non-existent on Yahoo. To sum up, a very solid defense, one of the best in the league, in terms of actual defense in games. In fantasy...top 10 or so.