I'll add a bit more to the thread instead of expressing my distaste for the product
As a mail client I would have to once again agree that the UI is absolutely atrocious, lacks certain 'common-sense' features that you would think would be in a mail client and has random quirks that make its usage kind of cumbersome. There are lots of really random bugs in 6.5 as far as the calendar goes and if people change their date on a meeting, sometimes it won't update properly, stuff like this.
This being said, its DB capabilities and programming with LotusScript really unlock this product's use, domino(server and messanger client), etc make up for these issues at least a little bit. It's very functional in the environment I work in and I pretty much use it constantly for scheduling meetings, organizing projects, having document repositories and things like this. I've worked in places with both Exchange and Lotus and while the ease of use of Outlook (though it's a wretched program) made tasks quicker for me in some cases, Notes is still better in my opinion.