Outlook takes to long to start up in the morning. If it could start instantly I'd walk into the conference room on-time, but instead I'm several minutes late. Now that I'm working from home it needs to get the chat client started instead, but the the end result is the same, if I'm not running early I'll be late to important early morning meetings. (my early morning meetings are with India, so they will quickly assume I'm not joining at all and head to their supper - timezones are such that the best time for everyone is my breakfast and their supper)

Outlook also has been chasing the flat look fad which is against usability. Most of the others are too so it is hard to fault them, but I still wish they would have the guts to say this fad is wrong...

Once they are all running and you are used to the ui quirks they all work well enough.