After some days using Opera 10 beta for GNU/Linux, I'm quite impressed. There is some features that I would be glad if they implemented (or at least let us implement, as for example an Esperanto dictionary for the spell check).
By now I didn't have enough time to test the main feature of this version, that is Opera Unite, but is a kind of web server that serves pages (by a proxy in opera) to the Internet. That is a nice idea, for example you can have (as a real normal user, or a terminal user) opera running and have some services (photo sharing, file sharing and other things). But for now I didn't see the real potential of that new feature.
But, the things that I can see is the normal Opera. The render is quite stable and the browser, after more than 5 days, it just consumes 2269M in VIRT (812M in RES), that is almost the half that ice-weasel uses in my host at the same amount of time (and having almost the same number of tabs open).
The only problem that I had was using some widgets and trying to use the scim input method in the X (for typing Japanese directly). Sometimes the scim program get lost in the windows of opera and I could not pass (using the mouse) from one input method to one other. But when I close the widgets, the behavior of scim becomes normal.
AS I didn't have some time to test this deeply, I cannot say if the render is OK or if it has some bugs (like some strange combinations of CSS2 and XHTML1.1). But the usual pages are loaded fast and well (or at last, faster than firefox and rendered almost equals).
Other thing that I remember that in some prior versions of Opera was the Google products. But now, all the products that I test (Gmail, G-Talk, G-Docs, G-Maps, G-Reader, Blogspot) runs OK and fast.
Al the rest of the good things on Opera are there, Opera mail, chat, booksmarks, etc. I do not use almost any of that, but are usefull for some people. And as I remember using that tools, were easy to use (at least for an advanced user).