I have both gmail and Yahoo accounts. I actually prefer Yahoo.
I didn’t know about that feature before now – too cool. I find gmail is way less cranky in receiving large emails to do with work than my normal addy – and that feature about receiving mail from other addresses and appearing to send from them also has proved extremely handy twice now when I’ve travelled out of town this year.
Gmail actually makes me nervous. I use Google as my main search engine, and I don’t want to make it easy for one company to link my emails with my searchs. I’ve had great luck with mail.com for almost 7 years now. They have popups and ads, but no spam and a decent filter takes care of the popups.
Just to let you know, Gmail does search your emails for keywords so it can target you with advertisements related to topics you discuss. They claim that they don’t use the information for any other reasons, but I don’t know if you would consider that too intrusive.
I’m with Tastes of Chocolate. I use Yahoo Mail because I think that Google would have way, way too much information if they had my email as well as searches. It only take a few minutes to sign up for a Yahoo address, so you can try it yourself.
I recently switched to Gmail after 10-11 years with Yahoo Mail, and I find that conversations were/are the biggest learning curve (for me). Labels are so similar to folders that I hardly had any adjusting to do at all, but thinking of e-mails as conversations instead of as separate messages is still hard for me every now and then: I’ll go to delete a reply and find that I accidentally deleted everyone else’s replies, too. 
Luckily Gmail makes that kind of mistake easy to fix, but it still takes some getting used to. Just my $0.02.
Gmail. There are tons of features, but the site is designed so well, you’ll never be bothered with any of them unless you want. Out of the box, it’s mind-bogglingly easy.