1. Both really. I've played GG1 for the PS1, GGX on Dreamcast, X2 Reload on Xbox as well as PC, and have pretty much been a huge fan of the series. Best 2D fighting series out there today.
2. It depends on the fighting game really. Some of them are easy to get into, Guilty Gear being one of them due to the fast pace, relatively intuitive combat system, and not too much difficulty. That said, easy to play, hard to master. Not all 2D fighting games are like that (CvS2's roster and Groove system make it harder to get into than it should), but GG's been good at keeping it easier than not. And no, not obscure gameplay, not sure what's meant by that reallly... plenty of people play the games and they aren't that hard to find. Guilty Gear's probably the easiest series to get into these days, it's easy to play, hard to master. Which is the way it should be.
3. Guilty Gear >> SNK > Street Fighter as far as series go. Slash >= Accent Core = Melty Blood >> Capcom vs SNK 2 > Garou > KoF series in general (with 98 leading the pack, probably) >> Street Fighter > Fatal Fury (too old, hasn't aged well, and everyone knows it as what came before KoF anyway). Would take too long to list all the series (eg, Darkstalkers, Samurai Shodown), but Guilty Gear has the pros on the gameplay, speed, and cast. Let's not forget music either, or that GG has a damn good storyline (only one better of the genre, Melty Blood) as a game, let alone as a fighter. AC tried new things and they actually *worked,* which is better than what I can say for both Capcom's as well as SNK's latest tries of fighters (though Neo Geo Battle Coliseum is awesome and also deserves to be on that list, above CvS2 but I'm biased to NGBC so maybe not). Anyway, this is long enough, point is Sammy does things which the others *should* do, and actually can pull off bringing new stuff to a series, while maintaining something that resembles balance. Well nowadays at least (no, everyone having an infinite isn't balance).