DLC was never really an appreciated word among the playerbase, even if it was justified. There's the Borderlands series DLC lists that comes to my mind when I think of that.
The first game had four, with a GOTY later released with all of them together with the main game, and is still for a good price now. The problem here was with the ending, which one had to look into the last DLC for that, but I won't spoil anything from it. That's a bad choice.
The second game currently has four main DLCs that tell their own tale, as the ending itself was satisfying by its own rights, and those DLCs actually add little to everything. The other DLCs of the second game are: A battle arena, two new characters with new playstyles, two level cap rises with a harder mode, and 5 miniquests. Clearly you can see that the second game did better on that topic, and you don't even have to buy them to play through the game. If you want the achievements/trophies/stackedshit, be my guest. As guessed, it also has a GOTY, with all of the above aside from the miniquests, but they are pretty cheap and don't really add anything so you can skip them if you want, but they can be fun too. The GOTY is priced like almost any new game, and with sales soon…
Speaking of that, you can challenge Raid Bosses, which are pretty much this game's Bonus Bosses. Without anything, you get one you can replay at anytime, and the second you have to be extra lucky and patient. Not every DLC adds one, and if you have a friend who has the battle arena, you can just play with them and get the rewards from there, saving you the money. The miniquests give one per each as far as I know, and one of them has the communities' favorite crab boss from the first game.
In short, if its fun to you, go get it. If it's cheap and fun, even better. I didn't put Borderlands 1.5 because… It's more of a separate minititle, like Farcry's Blood Dragon. If it follows the Borderlands current formula, think about it.