First let me admit that my current web browser of choice is Firefox, however I also have chrome and IE (and now opera) installed. Opera 10.0 was released today so I thought I'd take a look and see how it stacks up. The download is 6.6 MB, so not a bad size. Installation is quick, choosing custom just lets you change the install path and choose to create start menu, desktop, or quick launch icons. One thing to note that I installed this on Windows Server 2008 which only has the Windows classis UI, so buttons etc may look different.
The screen shot below is Opera 10 on Google's home page. One issue I noticed immediately is the address bar didn't span the whole width when I reduced the size of the window for the screenshot. This was remedied by expanding the size of the window which then showed the address bar and the search bar, and resizing the width by clicking between the two boxes. I think I prefer chrome's approach to having a single bar for both purposes.
Opera on the left, Firefox on the right.
I then opened 4 tabs with both browsers:
With Google.com, omaha.craigslist.org, Hulu (paused on a video), and eBay.com pages loaded. Opera was using 176,000K of memory, Firefox was at 187,000K. My machine has 6 gig of ram so even though those numbers seem high I still have plenty to spare on my system, they may be more frugal on system with less memory (but maybe not). After killing all the open tabs except Google, Firefox's memory usage dropped to 89,000K whereas Opera hung around 149,000K. Unexpectedly I did not have to install flash on Opera's browser when I went to Hulu. So I then went to a Silverlight site and didn't have to install it either. Nice.
Opera feels very fast when navigating. Pages load like they are in a hurry. Opening a new tab loads a speed dial window showing your most visited sites in thumbnails. There is a little x in the top right corner to remove unwanted items from the speed dial page.
One feature that only Firefox has implemented which keeps it at the top of my list is the ability to select a section of the page, right click and choose "View Selection Source" so you can see the html of your selection. This feature is also not available in Opera 10. New to Opera 10 is inline spell checking, so the browser will underline misspelled words in text boxes (this feature is available in Firefox).
I was unable to select gMail as my mail service when clicking on mailto links. The 4 options shown to me were Yandex, Fastmail, Opera Mail, and Mail.ru. I use (and have never heard of none of these). I've read other blogs that seem to show gmail being provided as an option. I don't know why it wasn't provided to me. I even went to my gmail with opera logged in, restarted the browser, and still no change. This will be a deal breaker if I cannot resolve the issue.
For the brief set of pages I visited Opera rendered all content correctly and quickly, and seemed faster than Firefox. However the lack of "View Selection Source" and using gmail as my default mail provider leaves Firefox as the reigning browser of choice for me.