The two first Borderlands DLCs are a whole lot of fun, and if you don't have either one and love the game, check it out!
The first DLC is The Zombie Island of Dr. Ned, which takes you to an island in Pandora infested with Zombies and ghouls. It's a very Halloween themed DLC, and it's pretty fun. The mood it sets is very different from the main Borderlands story line. It tries to set a more creepy mood, but in my opinion that didn't work out too well. It's not scary or anything, but what really saves it is the zombie shooting fun! The zombies are quite a bit more sneaky than you would think.. although they are pretty slow, they are not very noisy most of the time, and since they can pop out of the ground pretty much anywhere, if you are not paying attention they can overwhelm you. The story line is pretty funny, just more of that good old Borderlands humor. The environments look great too, giving a nice creepy feel to the place. Since this was the first DLC it's not really the best one, but it's worth playing for the zombie fun and the nice change of pace from the regular campaign of the game.
The second DLC in the pack is Mad Moxxi's Underdome Riot. This one is more of a 'fighting off hordes of enemies in an arena' type of game. You have 3 different venues to choose from, where you participate in rounds consisting of 5 waves of enemies. There are a few different types of waves. The first one is always a starter wave, where enemies are the standard bandits and skags. The other waves vary between Badass waves (where a lot of Badass enemies appear), Horde waves (consisting of only melee enemies like psychos and such), and Gun waves (where only gun wielding enemies appear). The final wave of every round is also always the same, it's always a Boss wave, where you fight a random boss from the original campaign of Borderlands. This is one of the funnest waves every time, fighting off old bosses is just great fun, and after defeating them, at the spawn point in the center of the arena, weapons and items appear. They give you 10 seconds to pick these and anything else up before you move on to the next round. Also between waves you are given 5 seconds to collect supplies of ammo and health that are dropped from the sky.
To add a little something to this gameplay, there are different challenges that happen at random. At the start of a wave a board appears with 4 slots, containing special conditions for that wave. The board starts with 1 special condition, and adds 1 more every few rounds. When you get 4 special conditions for a wave at the same time, the right mix can really make things interesting. These conditions very between having only one kind of weapon being effective (for example shotgun challenge), 'naked' (where you have no shields), special boosts for your enemies such as speed strength, shields etc.. and also conditions where critical hits deal more or less than usual damage. It's overall a really fun DLC, and it really accents the fun of multiplayer in Borderlands. There is also a weapons bank available in the DLC, where you can store those weapons which you love too much to sell, but don't want to carry around with you everywhere. This is by far the best feature here. The weapon and items bank is just awesome.
There are some down sides to this DLC though.. The amount of rounds you play in the underdome is not negotiable. You either do the normal challenges (5 rounds), or the bigger challenges (20 rounds, unlocked after beating every other normal challenge). There is no way to stop halfway and continue later, you have to play till you win, till you die, or till you get tired and quit. This is especially annoying when playing the 20 round challenges, because that one just takes sooo long! with 2 to 4 people you can pretty much kick ass in the underdome, but you usually end up quiting before you get to the end. 20 rounds of 5 waves each is just too much to do in one go.. other than that though, this DLC is great, and although you can't get experience from doing the underdome (sorry for you guys who planned on grinding), you will definitely find some great weapons in there. A lot of the weapons i use right now were found while playing in this DLC, and they weren't even given after defeating a boss, they were actually dropped by some normal lame enemies.
So in closing, if you really really like borderlands, and haven't tried these DLCs yet, check this out. It's really quite cheap and worth playing.