Just as good as PMAGs, but will work in a SCAR, HK416, etc.
^^ THIS!! ^^
Additionally, and while I have not yet gotten to test my newly acquired M3 mags out yet to give a seal of approval, the new Magpul PMAG AR/M4 M3 (3rd Generation, models MAG556, MAG550, and MAG557) PMAGs as seen in the link below (first couple mags listed on the page) have been adapted to run across the AR-15, SCAR 16/S, HK 416, SA-80, etc. platforms., thus eliminating the need for the EMAG more or less.
http://store.magpul.com/category/PMAG
As for "what are the really worth"...well...they are worth what the market will bear for them which I'm sure isn't what you want to hear.

Simply put, they are excellent quality magazines made by one of the premier manufacturers in the industry with a reputation for exceedingly high quality, function, reliability and customer service. They carry a premium (especially in these crazy times when demand is through the roof, production is as high as it can get at least for now, but the mfg is also about to start a multi-state move to GTFO of Colorado before its new draconian, 2A-violating laws go into effect, etc.). I'd say if you can find them for $25-$30/mag...they are probably a "reasonable" buy with any cheaper being a bargain right now. For example, a couple of my local shops have good stock of these mags, but they are starting at around $35/mag for non-windowed and they go up from there to around $42/mag if memory serves (thus explaining why they have stock). Just keep your eyes peeled as good deals on the EMAGs and other Magpul mags do come up, but you gotta be quick on the trigger to get them when they do. Get on the wait/notify lists at multiple vendors like MidwayUSA (has a 2x limit on most mags), DSGArms, Rainier, PSA, etc., as well as keep a watchful eye out here on the PX and on the BARFCOM EE Magazine section (there have been some sellers in recent days since the Feinstein bill DOA announcement have some M3 PMAGs for pretty decent prices...not MSRP, but not horribly inflated like GunBroker pricing). Good luck and God speed!