I'll agree with the above posters, most of the time the EP waits won't be bad but it really depends on how well the workers are moving the line and how busy the EP line itself is.
I was there the second week of November and the EP lines were terrible at first. After talking to some other people I found out that we just happened to be there right after the morning EE issues where the park had a power outage and so the parks had apparently handed out a whole bunch of express passes to the people who were affected those mornings. For the first two days of my trip it seemed like the EP lines were longer than the regular lines and in some cases they really actually were.
I watched a family walk in about 10 minutes after us in the King Cross station and end up line in front of us after the merge point. King's Cross station is always bad for EP cause the merge point is so early but this particular worker at the merge point didn't seem to want to move the EP line, she was not doing a very good job of ratios between the two lines. I've seen that happen before at King's cross and I think it is because the merge point worker can see the switchbacks for the regular line but can't see how far back the EP line goes. That's just a theory of mine though. This particular case above the EP line was backed up all the way to the place where they scan your tickets, whereas the regular line was only through the first switchback. Then the worker held the EP line over and over again to let the regular line through. It was very aggravating and had we realized what was going on we would have switched and gotten it the regular line.
Of course, that was a rare experience, there won't usually be that many people with EP. But either way my advice would be to not park hop too much. Try to do the train early in the day when the line will be less because not as many people park hopping and then stick to that park for the rest of the day, you'll save a ton of time and be able to do more.
