Why do these magazine demand a premium over Gen 3 20rd magazines? I have some that I wouldn't mind selling, and I was just wondering why they are more expensive.

Some people think they look cool and some people will buy anything that 'they don't make anymore'.

No idea either, had a bunch of the Magpul gen 2 straight 20s and while I had no complaints I don't understand the premium they fetch, sold most of mine for more than I thought they were worth so hope they went to a good home. I'm just as good with straight 20 aluminum & steel mags.

Most people out there prefer the Gen 2 mags to the Gen 3s, in 20 or 30 round models. Most people also just didn't like curved 20 rounders. The straight 20 rounders are Gen 1 by the way.

One reason the MagPul straight 20's are popular is they are the most technologically advanced magazine that is service rifle competition legal.  The rules require that magazines be straight, so the superior slight curve 20's are illegal.  So a competitor's options are GI 20's, or PMag straight 20's - and that's about it.  I use PMag straight 20's, and find them to be excellent mags.

Lazy, have you ever had problems with good g.i. 20s?

I've been using 'em for years, not hard like 30s, but I have never had a problem with 'em.  Even very old pre-ban ones that had some miles on 'em.  

To be honest, not really, but I have seen others.  I was at a match a couple years ago, the guy I was scoring for asked me about my PMag 20, as they had just been made legal.  I mentioned all the PMag sales pitch, on why they are better than GI 20's.  He sort of shrugged, and then rolled into his rapid fire.  And JAM - a round nose-dived into the front wall of his GI 20.  It was realllly hard not to be a smug shit .  In truth, if he had better springs, it probably would have worked fine.  Most people use GI 20's and are happy.

I will say this - GI 20's allow about 0.005 longer OAL, than the PMag, which sounds silly, but I'd prefer it.  But the better follower and better round retention under recoil in my opinion just makes for a more reliable mag with the PMag.  Going to matter even more in 2016 as they are doing away with Alibis now (thank God).  That, and they look cooler.  To diverge a bit, what I do now is load 2 in GI, and 8 in PMag, so that it's impossible to F'up and grab the wrong mag.  Because yes, I've done them backwards before - at Camp Perry no less.  Bam Bam, drop mag.. insert fresh mag..  why is my bolt still closed....  SHIT, Bam Bam Bam,  reinsert dropped mag, finish string .  Technically, I could have been DQ'd for a hot reload, but my scorer didn't care as the panicked activity resulted in no competitive advantage.  (This is why I am on a one-man campaign to lobby the CMP to allow curved 20's, so I can use clear Lancers 20s!)

Just out of curiosity why are the slight bent 20 round mags illegal in competition?

Thanks

NRA and CMP rules for service rifle call for the rifle to have the external appearance of a GI service rifle.  So 20"BBL, standard A2 external appearance, including magazines.   The 1990's are over, so they are finally relaxing many of the rules going into 2016 (final rules not yet published), and it is possible that the silly straight magazines only rule, will go away.

what was wrong with Gen one Magpuls?  All of mine function just fine.

Each revision has certain "improvements" according to Magpul.