Imagine you picked up a Samsung Galaxy S4 on T-Mobile today. The phone, which has a $579.99 list price, would cost you $99.99 down, plus $20 every month for 24 months, as well as an additional $10 per month for Jump. You're first eligible for a Jump upgrade after six months, and let's imagine you wanted to take advantage of it immediately. At that point, you'd have paid a total of $279.99 ($99.99 plus $30 per month for Jump and your standard equipment installment plan). Assuming you were buying another new, high-end phone like the Galaxy S4 without Jump, you'll likely be paying somewhere in the neighborhood of $600 for a new phone.
Obviously, you've paid less using Jump at that point, but there's a catch: T-Mobile will take back that Galaxy S4 when you upgrade. You don't get to keep the phone as a backup or sell it on the secondary market. For some people, this likely won't be an issue — messing around with selling phones on Craigslist or eBay can be a huge exercise in frustration. And Jump gives you insurance benefits as well. Given the fact that Verizon and AT&T charge $7 per month for comparable insurance plans while Sprint charges a whopping $11, the $10 Jump plan includes T-Mobile's insurance plan and its early upgrade benefits don't seem so bad. Particularly if you're the kind of person who subjects your phone to a lot of abuse (though you'll have to pay a deductible to replace a broken phone in all cases).
While there certainly can be an argument for Jump if you take advantage of it on a regular basis, things don't look quite as good the longer you wait. Let's say you wait a full year before trading in your Galaxy S4 — at that point, you'll have paid $459.99 for the phone and your Jump plan. That's only $120 less than the full retail price for the phone, and you still don't get to keep it. Sure, it's a pain to sell phones on the secondary market, but a former flagship phone can usually fetch far more than $120, even after a year of use. There's also the fact that flagship phones tend to be on an annual cycle — so you could get caught after six months having to make a choice between "upgrading" to a phone you don't really want, or losing value the longer you wait for that next phone to come along.
It's probably fair to say that Jump isn't always better than your other options for upgrading your phone — it's just different. If you're confident you'll take advantage of the plan twice a year, every year, and you have a bad habit of dropping and breaking your phone it's worth looking into. If you're the kind of person who likes to have upgrade freedom without the hassle of selling used phones, or if you're prone to breaking your devices, it's definitely worth a look. But if you just want to upgrade your iPhone or Galaxy device every summer like clockwork, you might be better off saving your $10 every month and just making Craigslist sales part of your yearly ritual.