Have around 20+ mods now, some of them add a bunch of new monsters to last 3 biomes, so that the game is playable beyond killing Yagluth. Others add new items, crafting from containers, fixing angle of attack, improving archery, and other "small" improvements to annoying issues. Then there are mods that simplify things a bit, like teleportable ore (or even teleportable wolves for that matter) - I started from scratch with mods and I'm not looking forward to hauling ore around the world again.
I don't use mods that make the game too easy (combat wise), on the contrary, I increase the difficulty through some mods - more stars, various buffs, increased stats to monsters and such.
I've completed vanilla, so there's not much to do afterwards, but I'm not done with this game and I want to continue playing it :) Some mods do quite a massive overhaul to the game, basically allowing you to continue playing while you wait for new content from devs, which I suppose we'll be waiting for awhile, so why not spice things up with mods.