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; no 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:

I like mine as well however the top round does back out a little and you have to push it back in sometimes.... not too happy with that part.

I am waiting form mine to hi the door...

looks good...

Good to hear. I picked one up when I ordered the new mags but have not messed with it yet.

thanks for the review

I want to pick a couple up and was waiting on a hands on review

Glad you found it helpful.

Been thinking about these. Where do you get them for $14.95? Do these work with all gen Pmags including the new M3?

Show stopper for me - my Arredondo coupler used with GI mags exhibit ZERO walking rounds.

That would be a show stopper for me too––-I've never had this issue   I'm wondering if the mag is bad as opposed to the coupler?

I would think that simply rocking the magazine in, from front to back, during the reload would nullify the issue as the magazine well would effectively reset the round.

Good info; thanks.

Sucks they won't work with 20rd mags... bleh..

Have you considered loading initially from right mag and moving to the left mag? This way you have a better purchase of the mag when reloading and easier access to the bolt release.

Indeed, you're correct.   I tried not to get too specific in the video and stated there are a few ways to set it up for that reason.   There are a couple ways you can use it and still have a good ability to control the rifle.   The downside I can see with doing it as you suggest is that it would be a little more difficult to lock the charging handle back should you need to in a double feed type scenario (still very do-able) during the 1st mag.  

Both ways would work just fine I suppose.   There's not always 1 right way to set it up but there certainly are wrong ways to do it.  

I use mine on my HD SBR.  60 rounds for instant HD if something happens, extremely useful.