You'd probably be fine stepping in with the new season, but honestly, there's only two seasons behind you and they're short. Definitely worth the watch. As I've said before, it's campy and fun with moments of brilliance, and it's worth it just for Jason Stackhouse quotes. Once the season starts, I wouldn't advise jumping in -- usually, there's an ongoing story for the season and it would help to have that behind you.

Hmmmmm. I'd say that, since you won't be watching it tonight anyway, go back to the beginning. It's a bit soap-opera-y, so there's all sorts of romances and allegiances and backstories that you'll miss if you just jump in for this season.

This show is so not srs biz. Like not even in a Doctor Who campy sort of way. Doctor Who always has this pulse of seriousness underneath all the crack and wackiness and cheese. True Blood is just 100% cheese, all the way to the bottom. It's fun, though. And there's a massive amount of attractive people having graphic sex.

True Blood is unbelievably tasty crack, it's true. But start from the beginning and watch the DVD extras - they're playing, in a very interesting way, with the vampire=queer analogy that's powered so much story-telling from Bram Stoker on. Plus they're taking the setting, Louisiana, very seriously, so there's some smart thinking about race that's gone into the series as well.

But, mainly, it's full of some very good-looking people in their underpants. So.

The series is based on books, so you could check out the books first to get a taste of the characters.
There are loads of fun characters to keep track of and each one has his/her own storyline. Each season is a story arc, season one had to do with a serial killer of vampire "fans", series 2 had to do with an ancient Earth goddess taking over the minds of the townspeople.
I just got HBO on Demand, so I had the fun of True Blood Overload and marathoned the first two seasons in one week. Quite a feat when you realize I can't watch it when the kids are awake/home. LOL
And yes, the story is about vampires, but the main character is a telepathic human. She falls for a vampire because he's the only one she can't "hear". All the rest of the characters are connected to her, or her friends.
So my advice is that yes, you should watch each episode in order, so that you understand how everyone is connected to each other.
When you do get to watch them, let us know who you like better, Bill, Sam or Eric?