I will say, though, your experience as a SPH will depend on the number of tickets you actually want to keep (because that dictates how many you will have to sell) and your ability to get good partners. I have never had a problem getting partners but my seats are in a section that is only available as a full season plan and has a SPH price that's significantly lower than the face value. I know folks who have seats in other sections that literally cannot give their tickets away for free. So I would start with some due diligence and see if there are a lot of people looking for partners in your potential section, or groups looking for members. It may be possible to get some (or even all) of these benefits by joining someone else's group.
I go to an insane amount of games. I am on track for 50 this year. So that makes it worth it to me. If I was schlepping in from Outer Virginia and only made it to 10 games a year, I might evaluate differently. My seats usually sell fairly easily, although its been harder this year between team suckage and weather earlier in the year.
I totally forgot to mention two of the biggest benefits that bluestreak highlighted...20% off on all merchandise and vendors sales and dedicated e-cash lilnes. If you eat and drink at the park, that can be serious savings. Also, the e-cash express lines are a godsend at times. And the SPH dedicated entrance can be incredibly useful on real big days like Opening Day, etc.
The dedicated lines are good sometimes. Season ticket holders tend to be older than the general public and sometimes the SPH entry gate gets gummed up with folks bringing in two weeks worth of provisions for a 2 hour game (Say what you will about milennials, they don't feel the need to bring 3 full bags of stuff to the park). That being said on super busy days it's worth it when they enforce it.
But the ecash lines for concessions are awesome. Especially at the Shake Shack where it literally saves you 2-3 innings of waiting time.
But no matter what, I will never wear those nerdy lanyards with the SPH cards in them. 
I don't understand why folks wear these. Is it bragging over seat location? Is it so if you get lost an employee can guide you back to your seat? The one purpose I can think of is so the person that guards the section can look without me pulling the card out of my pocket. But after like 3 games the guy knows me and never looks anyway.
