There are two types of Express Plus passes available for the general public. Express Plus passes are available for once through the line on all participating attractions...prices vary depending on the anticipated crowd level for that particular day. The other is the VIP Express which includes theme park tickets. VIP Express offers unlimited Express access which is what the onsite guests receive but does not allow any special access on Forbidden Journey, Hollywood Rip, Ride, Rockit or Pteranodon Flyers.
The main thing with the EPs is when you're visiting. If it's not a busy period, you can make do w/o the passes but if it's busy, then you should consider purchasing the passes...but only after you arrive and can see what the lines are like for yourself. There is no pass that gives you "front of the line" access...only access to the Express line.