I've been using Outlook by choice, well, forever. I won't bother listing all the reasons for this. It's powerful and it works well. And, if you are a programmer, you can go to town writing your own customizations with VBA.
I use Yahoo Mail online for most of my personal stuff. Out of all the online email offerings out there I have found it to be the most "polished" (if I can use that term) and outlook-like. I tried Gmail a couple of years back and just didn't do it for me. As an example, the lack of a real time preview pane was a huge deal breaker for me. Both Outlook and Yahoo Mail have this. Maybe Gmail has this now, I don't know.
The other thing about Gmail that scared the crap out of me was to watch as one of our clients had their Gmail account evaporate because their AdSense account got shut down. They were new to the platform and made a couple of dumb and innocent mistakes off the line. Google bots summarily shut-down the account with no recourse or anyone to contact. As their account got suspended so went Gmail. I'll let you venture a guess as to whether or not these people are using any Google products now.
Here's a feature I wish email clients would implement: Tabbed accounts.
I run more than one company and regularly monitor a dozen email accounts. Outlook and Yahoo Mail allow you to setup rules to deliver email from specific accounts into designated folders. With Outlook you can also choose to store the email in separate files. That's all well and fine, but the whole thing has the risk of becoming cluttered and difficult to use after a while.
There's an Outlook hack that allows you to launch multiple Outlook instances. This works very well as I can launch one instance that opens the email addresses corresponding to one business and another instance with a separate business. Having multiple monitors makes this even handier.
What would be even better is if I could do the same thing with tabs. I want one tab per business --or a user-designated category. Each tab would receive and send email from one or many email addresses. They would all be independent while having relevant sharing capabilities (copy and past, drag and drop, etc.).
It's about context switching.
With something like that I could have a single email client handle all of my business email as well as personal within one application window. That would be slick.