I've been using Google Adwords for a couple of years now and find it so much easier to use than Yahoo's version. Like everything Google you can adapt it to how you want it, everything's quick and automatic and it's really easy to use.
Did you know you can't stop Yahoo ads appearing on certain sites even if you want to? That means if a site is spamdexing by filling itself with adverts and no other content you can't remove yourself from it, that's up to Yahoo, in the meantime irrelevant visitors cost you money. OK you could stop your ads, refine them, adjust keywords, but wouldn't eliminating bad sites from your campaign be easier?!
However the one area with Google that's lacking is finding keywords.
Google keyword tool is handy and what i've been using for most ads but that was until i found something that was far more fun! - Quintura. OK it may be doing the same thing but with all the floating, moving words it gives you so many more ideas. So for a list of similar words then keyword tool is fine but for inspiration use Quintura. Then when you're bored of that just start typing in random words - I was amused to find that "Crystal Palace" also brings up "knitting needles".
Quintura also seems better for finding negative keywords - words to eliminate from your campaign that you don't need.
Growing strange cacti (Feb 2009)
Is the Google Keyword Tool any use for SEO? (Nov 2010)