Just wondered if anyone uses these - if so are there any disadvantages, or is it just all benefit ?

It depends on the application.

The ideal place that I can see using them is in a very short barrel(less than 9") that has a very high, short pressure spike.  The use of a pig tail gas tube will tame the gas spike down, but the due to the the gas pulse still being short, and the gas port being very large, the extra power spring will prevent the extractor from popping over the rim during the violent cycle.

The disadvantages are that extra force of the spring on the extractor may shorten it's life expectancy due to the added tension that must be overcome to allow the extractor to glide over the rim for engagement.

P.S. The SBR(9" and shorter) that functions with a gas system is a conundrum in it's self.  What started off as a 20" rifle that had variances built into it, has been stripped of these variable tolerance. By adding items such as a pig tail gas tube, heaver buffer, and stronger springs, you soften the gas pulse/bolt unlock time, and allow the SBR to function.

I put one in my rifle when I was having problems w/ ejection on my military 14.5" barrel.  It would fire and extract the round, but then it would try to feed the case and a new round at the same time.  Happened about once out of every 10-20 rounds.  Funny that steel case S&B ammo functioned fine, but brass case (S&B, UMC, IMI M855)  would always give me problems.  

Yes, the rifle was clean as a pin (as an infantryman I should know) and I even stripped the bolt to check the ejector and ejector spring.  Bought a new Colt extractor at $26 (whew!), didn't help at all.  Got one of the heavy extractor springs, and have not had a failure yet after switching to all brass case ammo.  Use S&B, Fed, but mostly the SA surplus.  No problems at all!

Thanks Josh

I installed one in my 16" - works great now. My 20" was working OK, but I installed one anyway. No problems. Looks like these $4 items are well worth the money