I am a great Opera fan. On my PC it is much, much faster than IE.
In fact I can open a page in IE, get frustrated at the wait, go to Opera, type in the URL and Opera will still get there before IE.
There are some pages with lots of cleverness that simply don't run in Opera (particularly swizzy .NET/Java Applet stuffy) so you need IE in reserve for them, but in 99% of cases Opera beats IE in every respect. It's not just the MDI, but the flexible navigation pane, the flexible searches, the wand...the list is endless...but above all, it either doesn't have the security holes of IE or (just as likely) the pathetic w@nkers who write worms and viruses don't aim at Opera cos it ain't so much fun.
As far as eMail goes, I'm actually quite happy with OE. It does everything I want and need, so I stick with it.
W