So many answers, and many very valid points. I have not read every post intricately, so if this has already been said, well, then I guess it is worth repeating.
Limping is really only good when you have a reason to do it. And if you are not sure what to do, and you limp, you did it WITHOUT a reason. That will always mean you are playing without thinking or having a plan. That is always an obvious indication that you are not playing your best.
There is mostly not one correct plan of action to take in a hand. As mentioned, if you know you will get more callers, and then be raised by a LP or Blinds player, a limp is a plan. If the plan doesent work, oh well.
It also depends on the stakes you play, and the history you have with your opponents. But, a rule of thumb, in EP, if you want to play a hand, raise. Limping and getting a lot of callers is the WORST thing that can happen with a big pair. Read post #7.