Opera has been a useful alternative browser for quite some time. I have used it right from version 5.x. Now that it has become completely free (it had banner ads earlier), it can be looked upon as a handy browser. The biggest downside to using this browser is its problem with some sites (both display & functionality), & this prevents it from being used as the sole browser.
Features: It includes some basic features found in most of the newer browsers, like tabbed browsing, popup blocking, integrated web searching, RSS feed readers, skinning etc. It also includes a basic download manager, mail client & ability to incorporate user scripts into viewed websites. There are also some nifty features for 
dictionary/encylopaedia lookup, web searches, go to URL for highlighted text, mouse gestures etc. You can also save different browsing sessions & launch them at the browser start or at any other time. It also gives you the ability to apply user defined styling for a viewed page. There are many more features which may not be used very frequently, but are quite handy nonetheless.
Pros: Opera has always been a fast browser, & it comes with a handy set of features by default. Users with a slow internet connection will especially benefit from its ability to toggle the option of displaying images on a page by just clicking a button. Another feature useful for people with small screens is the ability to fit the displayed to the screen width, though this may tamper the page layout (this can also be done by zooming in most browsers). The installer size is also quite small, atleast compared to the currently popular browsers.
Cons: The main problem with Opera lies in its incompatibility with some sites. Due to this the site display/layout may be messed up, or the site may not load at all. This fact makes it difficult to make Opera the sole browser on any system.
What next: The current version of Opera is 8.51, & the preview of the next version (9) is available on some sites. To check out the current version of Opera follow the below link:
To check out the preview releases follow the link below & select the OS platform:
AB's Reflections 
Opera is the best browser around except for a few sites which dont load java stuff properly and Im not sure about the security when transacting