DISCLAIMER: This is a forum, where I am free to express my opinion. If you don't agree...sorry.
Okay, staying relevant to the original topic annnnnnnnnd some of the posts I've read (and remembered ^^
If you have some 50s, you know where to go. If you don't, ask someone in your linkshell; if you don't have one, that is about half the problem. There was a full group in my linkshell that started partying together at...30 I think, and it just so happened they could all get on at around 6-7pm EST and grind until 10-10:30pm. They were all 50 in 3, maybe 4 days of "legit" grinding. So if time is the concern, a linkshell is a great thing to have (they all had 50s, but wanted to do it the right way, because it is more fun to them [and to me]).
Time? I pay college tuition to student teach from about 8-4:30/5:30 every day, have a part-time job, and pick up gigs on the side...not to mention my social and romantic life. Yes, I have PLd in the past, and even asked a LS member once to get me up to 35-40 so I could get a party quickly since back then no one really took parties below 35 consistently. But, I do have 5 50s, and the other 4 were pretty much all grind. So, I understand your time restraints, but it is possible. And if it's a class you enjoy, you should enjoy leveling it.
If it's not a class you enjoy now, why do you think you'll enjoy it for end-game? "I've heard X job is really good at Y end-game, so let me jack it to 50 real quick" is not really justifiable, imo. The class(es) I enjoy most, I enjoyed all the way from 1-50. Some classes have been used in end-game and changed people's opinions about how useful they are.
To the people saying "partying is boring, flameflameflame," then get a PL. This is a place to express opinions after all, and like Edgecrusher said, it really annoys people when this stuff comes up. I say to try all classes to 15, and the ones you like, grind out. Need an ability from another class? Grind it up or get a PL. I do feel that grinding to 50 does give you a certain finesse over the workings of that job that grinding 40-50 doesn't.
And finally, to those who say all the content is end-game...that really makes me sadpanda. The core workings of this game is to experience a storyline as your character progress, and then augment that material with end-game, which adds to the story (funny how that works). If you play just for end-game...you're really just playing for the bragging rights of having beat X or having gotten Ifrit/Moogle/Darklight Y. If that's the case, I can not be of the same mindset as you, for I feel you are not really playing the game. If that is all that is fun to you in this game, you are leaving out a lot of content, and you can really get that end-game experience in stand-alone games much easier. "But then there's no one to play with!" Well...I'm sorry, you can just stroke your own e-peen then (I know this isn't everyone who just enjoys end-game, but it hit a nerve, so I apologize. That statement is not intended for everyone; although I feel like those who call me out for it are the ones it is intended for...because SOMETIMES, that is just how forums work.)
tl;dr Enjoy the game as it is. Level what you want to level, not what you heard is good for end-game. It makes the grind much more enjoyable.