Depending on how deep your league is, most of these guys are worth a pickup.
I'm not very high on Miller, but there's still a chance he'll be able to turn his season around. We'll know more after a couple more starts.
Lieber is simply a solid pitcher. Yes, he's had second-half struggles in the past, but that isn't enough to scare me off.
Park is the one guy in this group that I'd avoid at all costs. The rumor that hecould be headed to Colorado doesn't help, either.
I've never been as high on Sabathia as some; I think he'll continue to mix brilliant outings with disasters in the second half. His overall numbers will be decent, but he can be a frustrating player to own.
Dempster is another overhyped player, but a change of scenery might do him some good. Don't expect miracles, though.
If your staff is already full of Pedros and Zitos, you won't need any of these guys, but if you have some holes to fill, I'd start by at least grabbing Lieber.