I never got used to Opera Mail. In fact, I am not sure if I have any more than a single account. As for the Mask as IE, I did not try the feature. Its failure may not, necessarily, be due to Opera, but rather to the websites. AOL and Yahoo, and I am sure VH1, i are using sniffers, and will not even let you through, whether you have the plugins or not.
As a comparison, I use FF1.5.0.4 with its IE Tab extention. Thus equipped, I can use it to do Windows Updates downloads.
As for the plugins, Opera8.54, since uninstalled, had only the Java files, but my 9.0 has all the plugins which I use in Firefox. For good measure, I copied/pasted most of them from the Firefox plugins folder.
But, sites like AOL Music demand Active X, and there is no Active X plugin which is compatible with Opera, AFAIK. There is one for FF1.5.0.4, and it does nothing for Opera.
If someone knows how to enable the ActiveX plugin, please let me know (Opera claims that the ActiveX plugin should be usable)
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.0.4) Gecko/20060508 Watersquid Firefox/1.5.0.4