I finished all of Borderlands 1's DLC last night (except Moxxi's Underdome, which is just an arena, as I understand it).
The Zombie Island of Dr Ned has the honour of being the game's first DLC. And, whats this? Characters!? Writing!? Its basic stuff, but considering that the first game is basically devoid of both, this may as well have been Citizen Kane. It also introduces a new biome to Borderlands - dark swampy forest - which might not sound like much, but considering that the base game (and frankly, the rest of the DLCs too) presents Pandora to be little more than deserts, and literal piles of trash (and frequently, both at the same time), this was a breath of fresh air, and actually quite atmospheric, too. Unfortunately, this additional variety does not really extend to the enemies that you fight during the course of this DLC, which are, you guessed it, Zombies. And frankly, zombies just aren't that interesting to fight. You've got your normal, slight jog, shambling zombie. You've got the one that spits goo at you, and slows you down. You've got the one that runs straight for you, and blows up. And the big one that has a lot of HP, and throws shit at you. Basically your normal "videogame zombie" variety, and it wears pretty thin after the first 30 minutes or so. It kind of outstayed its welcome, about half way through.
Next up is The Secret Armoury of General Knoxx, and this one is hailed as the best DLC of Borderlands 1, and one of the best Borderlands DLCs overall... and I don't see it. The writing is immediatly much more in-line with where the sequels would take the tone of the game, and Knoxx was well written and acted (and I wish we had more of him), but really, the first thing that I noticed about this campaign, was the sudden spike in difficulty. And not even in a fun way, but in a "cheese encounters or I will die in seconds" kind of way. The DLC is also BIG, because it tries to focus on vehicle combat and traversal, but vehicles in Borderlands aren't great, which makes it an odd choice, and it comes with the added drawback of needing to trek on foot for minutes at a time, if you find yourself without one. I did really like the timed loot run at the end of the DLC though - steal as much as you can in 3 minutes, before everything blows up. In the end, it is still leagues better than the base game, but the difficulty spike and large maps were a pretty big blow against this one.
Lastly, is Claptrap's New Robot Revolution (RR), which I actually played before The Secret Armoury (SA), because even though it is set after SA, RR scales to your level from 35, whereas SA starts at level 50. A weird consequence of this, is that RR actually spoils the ending of SA. It wasn't a surprising outcome, but it was low-key annoying. Anyway, I would actually argue that RR is the most consistent of the DLCs. It doesn't have the higher highs of SA and Zombie Island, but it doesn't really have any drawbacks, other than a single zone being an absolute nightmare to navigate. It was a fun premise, but I really feel like they could have gone harder on the theme. It was alright.
Overall, the DLCs are an obvious improvement to the base game, and they offer a taste as to where the tone of the franchise ended up going... but again, I just feel like Borderlands 1 is entirely redundant, and the DLCs don't really fix that. In the end, in a world where there are 4 other Borderlands games, you would probably have a better time playing any of those, instead.