A friend and I are getting together one of the upcoming days for some solid gaming. Borderlands is on the menu, but I think we'd like to get our groove on with some of all the new tasty DLC.
We've got Zombie Island of Dr. Ned - should we get some of the new stuff, or are they badly done money-machines put on top of the original game?
(For what it's worth, we're both around lvl. 45)

dr neds was ok. mad moxxi's was kinda boring. i say your best bet would be the general knoxx dlc.

I heard Mad Moxxi's Underdome was terrible. General Knoxx is really good, so get that.

I would go with Knoxx first because of the level cap, Zombie Island after since its fun but way easy, then if you're really hardcore about borderlands Mad Moxxi.

In my opinion, General Knoxx was the best addon, followed by Dr. Ned and then Mad Moxxi. In fact, don't bother getting Mad Moxxi, unless you REEEEAAAALLLY need the bank space...

I'd go with General knoxx as well, it's actually got quite a few quests in it, a level cap increase, a new type of gun rarity or something (Pearlescent) and Scooter is one of the main characters of the DLC so you'll get some laughs too

If you want a place to store weapons and do a lil competing then Mad Moxxi's is the best place. But if you want you to co-operatively beat the hell out of a boss then pick up General Knoxx. (Damn there are too many X's)

Zombie Island. General Knoxx isn't even playable until you're at level 50... well, it is, but you won't really get anywhere, since your weapons act like peashooters against the level 51+ enemies (on playthrough 2). Zombie Island will get you up to the level cap before you start General Knoxx, while General Knoxx will take you to level 61, making Zombie Island pointless afterward. 
 
If you get Zombie Island, at least you'll be leveling, whereas, if you get Zombie Island after General Knoxx, you'll be stuck at level 61 the whole time, making the DLC completely pointless. 
 
So, if you ever intend to play Zombie Island, play it now. Otherwise, get General Knoxx, and don't bother with the other DLC afterward.

You have to play through all of Mad Moxxie in order to get the 2 extra skill points right?

There's only two other ones. General Knoxx, and Mad Moxxi. So, definitely get General Knoxx.

General Knoxx, I like it more than the retail release

Gen Knoxx.  I haven't played it, but I heard Mad Moxxi is horrible so I'd suggest staying away from that one.

I just made the mistake of buying Mad Moxxi today...don't do it!! Zombie Island and General Knoxx are both excellent though (but Knoxx is admittedly a more significant package for the same price).