I'll share more info with regards to what Loo has said.
My WHOLE collection of only 4 GS is pathetic.. Its not a collection, just a few purchases.
Based on the GS site, ALL current GS Autos are single barrel 72 hrs except the Hi- Beat which is 55 hrs.
2 of my GS are Spring Drives and the other 2 are a 130th Anniversary handwind simple time only watch and the other the 36,000 Hi- Beat. The date on my mechanical GS hi- beat start changing at 11.35pm and by 12.01-12.04 there is a click and it changes completely over to the next day.
My 2 Spring Drives are even better, they start changing at abt 11.40 and completely changes over at 12.01-12.02 with a click.
Perhaps Loo's experience is with the older generation GS. Also I cant comment on all other GS other than the ones I have.
Its true however that there are GS and there are GS. My advice is that if you want to get a GS, go only for the top Master Shop models which are higher grade GS. The prices will also give you an indication.
Base GS can be got for abt RM 10-12k (SD or mechanical auto) Quartz GS is even cheaper RM5-8K.
Master Shop GS are between RM 18-100k.
I agree with Loo that not all GS are top notch. After all you get what you pay for.
Personally I only bought the current 4 models as they are the only ones I assessed were very good quality in terms of all the aspects I was looking for and having specific features such as Hi-Beat, Spring Drive, limited hand wind calibre, etc..
Loo is also right that there is no micro adjustment deployant but my SBGA031 titanium diver has a wonderful, very easy to use, adjustable deployant clasp. I was told that this was introduced earlier than Rolex Sub's glide lock thingy.
Loo is also right abt the general quality of Seiko leather which does not match the Swiss. But thats where you need to be very selective. My 130 Anniversary limited hand wind edition cmoes with a lovely lizard skin strap that is so supple. Googling the net on lizard skin, I found that the quality of that strap is the highest lizard grade based on the texture of the skin. The smaller the rings on the lizard skin the higher the quality.
As for deployant clasps, so far the 3 GS on the metal bracelet deployants has worked perfectly. I'm sure there are lemons but the onus is on the buyer to test kau kau before buying.
Loo is also right that part of GS allure is in its scarcity. I wont even use the word 'exclusivity'. Only a few GS models are limited edition but annual regular GS production also runs in only tens of thousands instead of millions but more because Seiko does not have the capacity for these labour intensive GS models instead of the automated production line models like its Seiko 5s, Prospex, Brightz, SARBs, etc which are churned out in the millions annually.
Hope this helps.
I am really ignorant in the field of Seiko. But I am just wondering, how much step up is there from a JDM seiko and a Grand Seiko. How big is there a difference from say a MM300 and a Grand Seiko? Does the difference in the price commensurate with the difference in say quality? Complication? Also, I was told that Grand Seikos can only be serviced and repaired in Japan. Is this true? Thank you for any feedback on the matter.
Unfortunately, my only experience with a modern GS is the SBGM003 GMT Automatic. The watch was my brother in law's and I held it on ransom for a while because he borrowed my Rolex. 
When it comes to GS, I guess JOS is THE man you'll need to speak to. If you are nice to him, he will also bring his whole GS collection for your close and personal examination..... 
Basically, I found the finishing of the SBGM003 to be excellent whether you're referring to the case, the dial or the hands. The crown winds super smooth. An almost perfect dress watch. Time keeping was almost spot-on regardless of the position and regardless whether it's resting or worn on your wrist. The SBGM003 does not have a see through caseback, but you just need to google and find pics of the decorated movement. IMHO the watch is definitely comparable if not better than say a Zenith Elite, a GO Senator or one of those entry level JLCs. The SBGM003 has about 40hrs ++ of PR, but Seiko continually improves its products, so this GS GMT automatic (now renumbered SBGM021) now comes with a superior mainspring which gives it 72 hrs of PR using one barrel. I could be wrong, but I think most if not all new GS autos have at least 72 hrs of PR.
Now to the not so good news. The date change is pretty much like any normal seiko mechanical. The date starts to shift at close to 12 midnite and the shift to the new date is completed gradually after 12 midnite without the usual "click" we find in many Swiss made watches. I know most people do not mind this, but this troubles me a little. Next comes the 19mm wide leather strap. Quality is good but not super. Anyway, Seiko is not known to provide good leather straps generally.
Overall, like most Seikos, I guess you'll get a watch which gives a real bang for your money. But I believe many people may be attracted to owning a GS due to its aura of novelty, exclusivity and perhaps even mystery. These watches are after all not that easily available outside Japan, especially the GS Master Shop selections which are (as I understand) a step up from the "normal" GS. As usual, adviseable to do lots of research before making the purchase so that you'll have a more balanced and informed view. Many watch sites may (I suspect) be guilty of giving overly glowing reviews. If you mine the internet far and deep enough, you'll find complaints about the GS, for example, no micro-adjustment on bracelets, the GS Spring Drive SBGE001 not able to fully charge, deployment clasp problem etc.
The MM on the other hand is a watch designed for the water. Its 8L35 movement is (I believe) an undecorated GS movement. It is definitely a few notches higher than the "Sumo" and far superior than the normal SKX007/009 but finishing wise it's definitely not GS quality. You don' even need a loupe to see unevenly applied lume on indices. Some pieces have bezels that turn too easily, some have bezels that are far too firm. The problem comes from the resistance spring underneath the bezel. Some are bent at a far too steep angle, making the resistance a little too high.
Sorry, I have managed to rid myself of the seiko-fever a while back, so my knowledge is no longer up-to-date. You can get more information from the folks at the SCWF but they generally tell you all that is good when it comes to the GS and the MM 300m.
