From a .hack fan, this may be biased, but I really liked the .hack universe.
Info on Project .hack:
Project .hack is quite a large franchise of games, mangas, anime and novels. the list (in order of timeline) comes out like this:
.hack//A.I. Buster (Novel)
.hack//Sign (Anime)
.hack//Liminality (OAV with games)
.hack//Infection (Game series 1)
.hack//Another Birth (Novel)
.hack// Legend of the Twilight (Manga/Anime)
.hack//Roots (Anime)
.hack//G.U. (Game series 2)
+ Soundtracks/Minitures.
Although, how you get them and their availability now is up to you. I've not seen any .hack games for Infection for a long time, and G.U. is probably a US release only (outside Japan).
How good is Project .hack?
Imence. The series are usually heavily thought out storylines with action plotted in different places. It's not an all-out fighter series. It concentrates on lives in and out of the MMORPGs, which I thought was an obvious but nice touch. The best thing about .hack//SIGN was the music. That and the story worked well. .hack//Roots had ok music, but not my favourite soundtrack.
The books were an interesting read too. .hack//LOTT has a different story in the anime to the manga, but I found the manga was more interesting. You don't have to be computer literate to understand what they talk about, and I like the art in the manga (and the odd pics in the novels).
Overall, I rate this highly. It's not action packed and grewsome as other animes/mangas (like GITS), but the voice actors are good, the art's nice and the stories in both are great. The 1st game series is quite repetitive in gameplay, but if you do become devoted to this, it's still worth the buy for the story. Don't know much on .hack//G.U. though.
Hope that helps.