I voted it depends on the situation, in a normal game I use them but obviously for challenges I don't.
Mainly I use Twallans mods, quite a lot of them, a few are must haves though for me, Master Controller, Register, Story Progression, Overwatch, Error Trap and Dresser. The last one has saved my town from turning into a mix of china and egypt in the fashion stakes, before it I could pretty much guarantee every person who aged up would have an outfit from World Adventures on and a silly hat, drove me mad, I used to change their clothes with Master Controller now I don't have to as Dresser does it all for me.
There are three other modders who I also use quite a few mods from, Shimrod, Bluegenjutsu and Twoftmama. While Twallan saves my game with his bug fixes these three save my sanity with their annoyance removers. I find if a sim is overdoing an interaction they usually have a mod that will reduce or stop it altogether.
While some things I prefer sims didn't keep doing there are others I wish they would do on their own, mods solve that for me too, with a combination of Woohooer and one of Bluegenjutsu's romantic autonomy mods I can just sit back and let them fall in love all by themselves without me lifting a finger, to see them go from flirting to one of them confessing attraction, doing that first kiss on their own and even proposing is wonderful. It has on occasion got me in trouble when a sim has got involved with more than one person autonomously but it adds to the drama and fun.
But despite all that when playing my townie dynasty there was just one thing I really missed having, a story progression that kept the original town going, I hadn't used EA's story progression for a long, long time, I forgot how utterly random and nonsensical it could be, moving people, killing them off, or replacing a perfectly good interesting family with a randomly generated batch of clones. But even so it's still a blast to play and proves to me that I can indeed play without mods I just prefer not to.