Outlook doesn't suck. Most businesses love Outlook. Consumers with light requirements hate it mostly because of its complex nature.
Anyone with power user requirements loves Outlook...
I love the integrated power of Outlook from a feature standpoint (with some significant misses). But the UI and the programming do suck. On Windows at least, it's the most single-threaded piece of crap I've ever seen on a full-fledged computer. Any resource contention issues, and the app just locks up and freezes. Combine that with Windows general lack of a useful window manager (ironic, that), and you get interesting screen artifact hijinks that you can easily use to laugh at the product.
And MS owns the entire stack - both Outlook and Exchange.
Oh, forgot one... Maybe they've fixed it recently, but on Outlook, you can import vCard files, but Outlook stops processing data after the first vCard in the file!!! Makes it kind of tedous to send contact lists from your Mac to a friend/colleague using Outlook. Seems like a rather obvious use case. Sigh...