I've read all five of the books in the series so far (#6 comes out in November) and they are definitely young adult and I did enjoy them but definitely not as much as the Twilight series though. The books mostly take place in a kind of high school for fledgling vampires once they are marked so most of the characters are around 15-17 years old. There is kind of a love triangle to the story but it's not really like the Bella/Edward/Jacob triangle and a lot of teen "slang" is used throughout the books when the characters talk which I thought was a little annoying after awhile but then I'm almost 31 lol. Overall they were decent reads though.
Here are a couple of other YA series that I liked even better -
Mortal Instruments trilogy by Cassandra Clare - City of Bones, City of Ashes & City of Glass
Wicked Lovely trilogy by Melissa Marr - Wicked Lovely, Ink Exchange & Fragile Eternity
Blue Bloods by Melissa de la Cruz - Blue Bloods, Masquerade, Revelations & book #4 comes out in October