For those who don't know:
Shōnen anime is anime for boys under 18.
Shōjo anime is anime for girls under 18.
Either genre has a huge target range, and some titles classified as one of these are R-rated content. Shōjo not so much, but it's possible for either. I'll give equivalent ratings for Americans for each show as I go along. These ratings are my own and just an estimation of where I think the MPAA would likely rate these shows.
Over the last week (Starting and ending on Monday) I have watched well over 48 hours of anime. Apart from revealing my stupidity, I'm unsure as to what this says about me. Blood type: O+. Astrological sign: Taurus. Zodiac sign: Dragon. Height: 5'10". All of these are important things, OK? Also, this post is resembling a bad personals ad more and more. Moving on... the shows I (will admit to having) watched are BLEACH, Welcome to the NHK!, and uhh... Skip Beat!.
BLEACH (PG-13 for violence, blood, partial nudity, and some sexual themes) (Season 1 is borderline PG) was pretty awesome. For the first season. Then they took Rukia the Awesome away from me and I watched the next 2 1/2 seasons waiting for her to come back and make the show cool again. That never happened, and the show descended into a Naruto or Dragonball-like power escalation thing (There's probably a term for it, but I don't know what it is because it's dumb.) which I am told never stopped. Basically, the producers said after season 1, "Oh, we've gotten all of that useless 'character' 'development'. Now we can get to the stupid typical shōnen fare." The series is ongoing, and they haven't stopped that stupid powerup crap. Apparently.
Verdict for BLEACH: Watch the first season because it is awesome, then never ever even think about looking at the rest of the series. (Then again, if they do stop doing the idiotic flexing-mah-muscles stuff, it may be worth it to start watching again.)
Welcome to the NHK! (R for nudity, drug content, and thematic elements), also classified as shōnen (though thankfully devoid of fighting for the sake of fighting), was an incredible show. Excellent soundtrack, realistic take on life including very human characters, and a talking appliance every now and then. The female lead was the highlight of the show for me, as she's always so nice and patient with the jerk of a main character. I'll admit to crying several times during the show.
Verdict for Welcome to the NHK!: It's worth watching. You may not like it much, but it's certainly different from most shōnen. I liked it quite a bit. (The animation is bad at times, but nothing anyone who sat through The Hobbit can't handle.)
Skip Beat! (PG for because it's really tame, but there may be something I can't remember so I'm being safe and not giving it a G) is shōjo. It is hilarious, though I'm unsure of how funny it's really supposed to be. Character development is pretty good, as it's for girls (most girls care about that more than boys). The plot is different from what I'm used to, the main character is a girl, and some moments feel a little strange for me. The acting-based action is weird. It's treated the same as a fight would be in a shōnen. But all of this is beside the point. For reasons I find hard to express, I LOVE LOVE LOVE LOVE LOVE this anime. (My brother admitted to liking it as well, so it is possible that other dudes will like this. My other brother and sister are kinda meh about the series, so apparently YMMV.)
Verdict for Skip Beat!: Basically the best show ever. Seriously. (Easily my favorite anime of all time. Perhaps because I am an idiot. Discuss.)