I've tried a few mag couplers over the years and my favorite to date is the Magpul MagLink. Here's what I've found while using it over the last few months:
Pros:
-Durable. This mag coupler has held up to banging around in my range bag, swapping mags many times, and all the shooting I can dish out to it without missing a beat.
-USA made
-For those living in states that don't respect The Constitution with their magazine capacity limits, it gives you a way to have an extra mag at the ready.
-Works with both the MOE Pmag and the GEN M3 Pmags
-Easy to put together
-Can be configured in a couple ways so it doesn't interfere with manipulating the controls of the AR for both right and left handed ARs
-Very lightweight
-Cost. I picked it up for $14.95 shipped
Cons:
-Weight. Having 60 rounds hanging from your rifle increases the weight; not getting around that.
-Some people that have used it think changing mags is awkward. I haven't found that but your mileage may vary....
Here's a video with some shooting, mag changes, and a discussion of the products hits and misses:
MAGPUL MAGLINK HD VIDEO REVIEW LINK