I've never use the clamps or couplers that join two 30rd magazines together but am interested in getting a few (if they work well). For those that have used them, please tell me how they perform and which type/style/brand you prefer.

I was thinking of them along the same lines of a spare shotshell carrier; another way to keep extra ammo at the ready and affixed to the weapon. Some of the ones I saw appeared to have a screw in the center to tighten them; maybe a type of screw head you could use a penny or something similar on? A thumbscrew would be even better, but I haven't seen any with those.

Gimmicks IMO. If you want 2 mags, install a Redimag.

That's a good option too, thanks, I was unaware of those. I found this video that shows them: http://www.youtube.com/watch?v=Moas5mUD9yA
However, I prefer not to mod my rifle like that.

No problem raf. I thought these things were more commonly used than they are, it appears. I've seen the the spring steel ones for as low as $4 so it's not like I'm trying to "bling" out my rifle. The spring steel ones may be shit, I don't know. Maybe there's a better one or maybe they're all shit?

What important need does this contraption meet that cannot be met in some other way? Well, I've already explained that to the best of my understanding but was looking for input from other who've used them. I've also seen (on TV) three-gun shooters using them with both mags as a sort of bi-pod or shooting rest when in the prone position.  

Yep - no substitute! -

no mods to rifle needed - installs in minutes -

When I was in Iraq, I had a "high speed, low drag" setup of something like this. I was there in 2003 and we really had no idea what to expect or what we would be facing, so I would take two mags and lay them on top of eachother and the one on top face it backwords! Then flip that mag long ways and slide it about 4.5-5 inches down so it would clear the mag well and put black duct tape to hold them together. For quick mag changes all you have to is hit the mag release and flip your mags real quick! Not the greatest of systems but works!

~Zach

No modification necessary.  They just bolt on. I use the standard old school Redimag which can be found on eBay or the EE for $50.

Actually I have found the setup to be very useful myself (mag clamp).  I only use one which is always in the gun (never touches a mag pouch).  Use what works for you, don't rule rule it out until it has failed...

There's a bunch of different ones out there - most are crap. In particular, the ones that just wrap around the mag are prone to sliding up and down which is an enormous pain in the ass.

The only ones I've ever owned that were genuinely good where some pressed metal ones that I got at a surplus store they held the two mags at the base, base plates together with the mags angled slightly apart and staggered. Much smaller package than the ones that hold the mags parallel and no slippage possible. Don't know for sure who made them but I was told they were Isreali... Never seen any since though.

Whatever you get, make sure that it fits your mags really tight (not all mags are exactly the same dimensions outside of the mag-well. As has been mentioned, you may also want to look at the redi-mag system which is more expensive but more versatile.