Funny...I have found myself to be a pretty steady Yahoo user on my first go through. Being that my nonprofit focuses on my ability to do quality research online...I am on all of them at some time or another and often for the same search...but I find I hit Yahoo first and compare from the others.
I like Ask for when I have NO EARTHLY idea how to begin searching for a topic or idea I have in my mind...things that for whatever reason I can`t narrow (automatically) to a few keywords and feel I need to ASK the question.
I personally am not in love with any of them really. I find that often switching back and forth I find some things on one that I didn`t find on the others...or vice versa. Not to mention...I think your mind gets use to the organizational style of one site over the other...like me and Yahoo. I personally wouldn`t say it was any less or more of a site than the others. I think maybe Google has just become more used because it is better marketed. I mean really...who doesn`t "Google" something. I use Yahoo and I STILL say I am going to Google it. Nobody ever says "I am going to go Yahoo it!". Like Mapquest...you Mapquest it...you don`t Yahoo Map it. Of course...anymore...even mapquest has been pushed out by Googling (hahahah...sounds like something sexual...hmmm...wonder if that is why everyone likes it so much....Freudian maybe).
Anyway...I just think it is a bit of preference...and good marketing that has put Google at the top of the list...and yes...of course...as they become more popular they ultimately receive more tie-ins with sites and such...which makes their search results more effective. All relative I think.
Just my humble opinion. 