Quoted:


the 'standard' is 20 round mags because for most shooters in prone position the 30's hit the ground. if they don't hit the ground, they don't have enough space to drop from the rifle without the shooter breaking position in rapid prone. however, 30 round mags are legal for competition.



20 round mags don't hit the ground, and when doing a mag change generally don't require you to do anything but hit the mag release to get your new mag in.

Quoted:
couple new p-mags came in, so i used them in a cmp match tonight, and i couldn't get the mags to load on the first try from a closed bolt. still got all my shots off in time, but was a little rattled...

PMags aren't a panacea, but they are my favorite.  Compare: GI mags are as cheap as you can get and still be one step above disposable.  Such does not fill me with a comfort.  

However, there are some things to be aware of with PMags.

1) The OAL is less generous.  Only a high power long range rifle shooter will ever experience this.  But if that's you - you like to load your rounds as long as you can and still stuff them in your magazine.  Which means you probably use an OAL of 2.260", or maybe even push it to 2.265".  GI mags can work with that.  Loads are often +/- 0.005", meaning 2.265" OAL can mean 2.270 OAL".  2.270" will bind in a 20 round Pmag.  Easily managed by just making sure your loads are all under 2.260"  (which means average probably should be 2.255").  To repeat, you will never encounter this as an issue unless you already have tuned long OAL loads that you use - which only a long range high power shooter will be doing.

2)  They don't always drop free.  Fixed by just filing a few places.  Lame that you have to be the one to do the filing, but the plastic files easy, and such is life.  Not all of my GI mags are drop-free either, so it's not as black-and-white as some people imply.

3)  The injection molding seam on the back of the mag, can, in rare cases, bind with the bolt hold-open catch.  I had this happen to me once - manually engage your bolt hold open in a mag-free gun.  Then insert an empty Pmag and try to drop the bolt.  By manually doing it you might have positioned the lever towards the forward face, and so engaging more closely with the back of the mag.  It can then bind with the seam when you try pushing the lever.  Easily correctable by smoothing out the seam with a knife or sandpaper.  Easy to do, but unfortunate that they leave the factory that way.  

4) Overloading.  In order to make it easy to insert a loaded 30 round PMag into a gun with a closed bolt, you need additional compression room for the bolt to compress down the rounds a little further.  GI mags don't do this well, which is why it's a bitch to load a GI 30 rounder into a gun with a closed bolt.  Usually such won't latch and the thing will fall out - not a good thing.  Magpul decided to make theirs have a little extra compression room, so that you can drop a partial mag from a loaded gun and a closed bolt with one in the pipe; and insert yourself a fresh 30 rounder on the fly.  Great, but that extra compression room also means that if you're not careful, you can accidently load 31 rounds into the magazine back home.  31 round loaded Pmags will jam.  For the unfamiliar this is a very easy mistake to make if you're not carefully counting your rounds.  The trick is to make sure the last round is on the right.  If it's on the left, you have an odd number of rounds loaded, which = 31 if you're bottomed out.  The PMags aren't marked to warn you of this, so while some AR15.com computer commando's scoff at fools who do so, I can see it happen.

So why get the Pmag at all?  Mostly because the "issues" above are non-existent for most users, and trivial/easily correctable for those unfortunate few.  On top of that:
1) It's a solid mag.  Just google some of the vid's, it's an extremely tough, and abuse resistant magazine.  Well above all others.  Being able to drive over it with a truck where all others fail, while obviously a showmanship ploy, does imply it's a pretty tough mag.  I want a tough mag.
2) Superior internal design.  The no-tilt follower is excellent (yes it, matters, my county Sheriff's office has switched all carbines to 20 round GI mags, because their 30 rnd GI mags jam too much.  They should have gone to Magpuls).  I'm also a huge fan of the bullet-tip channel in the front of the mag.  This prevents rounds from jarring around in the mag during firing.  There's an X-ray video out there somewhere of this - you can see the rounds in the GI Mag shaking all over the place, while the P-mags are held in place better.
3) Window - yes, I like it.
4) Cost.  Magazine prices are trivial.  GI mags are effectively nothing.  Pmags are twice nothing.  It's insignificant unless you plan on buying 20 of them.  Most people don't buy 20 - more than 4 is pointless for most of us.
5) Magazine cover.  I like pre-loading my mags, snapping on the cover, and not worrying about if the mag's stored in a dust free environment, or if the spring tension is going to create lip strain.  The cover relieves the pressure from the lips.  It's tidy.
6) Speed loaders - still work, they have a special groove in the back so that the GI stripper clip adaptors still work.  This is how I load most of my PMags.  Stripping in seems to work smoother on the Pmags than the original GI mags in fact.
7) Color.  I like OD green.  I can get my Pmags in OD green.  This makes me happy.

Don't forget to pick up a  couple of 20's too - they work great for the bench.

What do I do?  I only use PMags now, and if someone will trade me 2:1 my old GI mags for used PMags, I'd take that trade.  At Camp Perry this year, I saw at least one Rattle Battle team using PMags exclusively.