I am going to break from my usual and respectfully disagree with Smash and Vene. I think a large part of the irritation and disdain (which I share) for JEMS is the "J" in front. As Jambi mentioned it is certainly not a journal and it draws much ire by claiming to be one. That being said there are actually pretty decent articles in there from time to time. I just read an article on therapeutic hypothermia that was both well written and informative. It also shares news about what's going on in the EMS world, job postings, EMS related political news, and up and coming EMS related medical stuff (usually far later than peer-reviewed journals but still there). Also, if you are having a hurp-durp day and want to look at pretty pictures of ambulances, uniforms, or EMS equipment JEMS will absolutely let you do that.
With all that in mind I would still recommend not purchasing a subscription. JEMS and EMS Insider are both trade journals for light reading and your money is better spent on a peer reviewed journal, like one of those mentioned in this thread, where you are more likely to learn something. If NAEMT lets you get EMS Insider for free then that's your best bet. while I usually don't recommend giving money to such a worthless organization, if you wear 5.11 stuff NAEMT gives you a huge discount that will pay you back if you buy at least one pear of pants a year or a couple of shirts.
In summary: don't pay money for the magazines, if you can get your place of employment to buy them then I don't think they are a bad read or big IQ black holes. I would rather someone read JEMS than nothing at all, at least then you might not be stuck in the dark ages of EMS like many of our "colleagues" in the US.