I've flown through EWR and PHL a number of times. I actually did some of these flights on USAirways and I am sure you are probably amazed that I am still alive but it's true.

EWR is HUGE and PHL has five different concourses, too. My experiences at EWR when flying in from out of the country have all been positive. A lot depends on how many other flights are arriving at the same time vs. the number of Immigration agents on duty. The other potential delay is the one back through Security and let's get VERY REAL HERE: a lot of those Security delays are caused by fellow passengers who suddenly forgot to take the things that look like guns out of their carry-ons, are wearing enough gold chains through that scanner to send it into outer space, etc. IOW it is a crap shoot.

Your 59 minute layover outbound should be enough time especially if your arrival is on time IMO.

PHL...only time I had trouble was when someone's baby stroller straps got caught in the luggage carousel apparatus and we waited, and waited, and WAITED for the repair and the rest of our luggage to appear.

If I were you I would look at the on time arrival stats for the various flights especially into Newark. I find Newark is usually delayed and it's just a matter of course. especially in the mid to late afternoons flights are backed up.

I personally wouldn't risk Newark. Not so much about customs just about the general airport and it's horrible delays.

59 minutes isn't a lot of time - remember your flight to SXM will begin boarding 30 minutes prior to departure so your "real" connection time" is somewhat less than 59 minutes- but it is doable IF your inbound flight from SEA is "on-time". The problem, as williamscb13 points out is inbound flights to EWR are often put in holding patterns due to heavy air traffic.

If it were me I'd only take this connection if there is a viable alternative if you are delayed and miss the connecting flight, otherwise you just might be spending a day of your vacation at BEAUTIFUL EWR.

Last hint, Murphy's Law of Airports ALWAYS Applies - It says the shorter your connecting time the greater the chance is your inbound flight will be delayed, the departure gate for your connecting flight will be as far from your arrival gate as the physical layout of the airport allows and the TSA security lines will move at a snails pace. Of course, if you have a long connecting time your inbound flight will arrive early, your departure gate will be just across the concourse and your outbound flight will have some delay in taking off - all combining to give you plenty of time to sit in the terminal and enjoy BEAUTIFUL EWR.

As Dukey says, If 3 inbound overseas flights land within 20 minutes, there is a huge delay getting through Customs.
We came back LIS>EWR>PIT At least 1 other international flight landed within 10 minutes of our landing. Running 550 people through Customs in a reasonable time is a real challenge for the Customs agents. As I recall, the cargo hold luggage was transferred from plane to plane without our touching it. They may have asked 1 question about our carry on bags.
We did get on our plane to Pittsburgh, just barely.

you could apply for the global entry program if you are a u.s. citizen. don't know how long it takes to get clearance but it allows you to bypass the lines at customs and immigration.

but it still doesn't eliminate the problem of missing your outgoing connecting flight at Newark.

I would never risk a 59 minute connection through Newark. Are you sure this is legal?

Have you checked the on time record of this flight - if it's any time after about noon the chances of it landing on time are not great. And even if you do make it - your luggage my well not.

If this is a single ticket and they say the time is legal - they they are responsible to get you on the next flight out. Have you checked when that is? Would you have a delay of a couple of hours - or would you lose the whole day?