Opera has big potential but it will never beat that Firefox is easier to learn for average user. I've started my "adventure" with it since 6.x version and the only thing which always come to my mind after finals is: they should kick out the person responsible for marketing.
Does anyone ever seen releasing final version just two days after RC? Being honest: in reality it is less than one day because the rumour about RC has to spread out.
Opera has a lot of features that nobody knows. They focus on adding features but rarely manage to finish them completely.
Example 1:
AFAIR Opera offered filtering addresses via urlfilter.ini file. Later it evolve to RMB option called "Block content" BUT usability of it is questionable. Only real geeks know that this one exist. Is it so hard to give visible toolbar button for it by default? This way users could learn about pretty quick.
Example 2:
Synchronisation tool could be killer feature but they managed to break this one too. There is no possibility to decide which way your bookmarks should go.
For Opera to send your stuff to the server, it goes through all your bookmarks and speed dials, puts them in a custom XML format, and sends it through a secure (https) connection to the server. The server will then store the items in its database. When another Opera with your login name comes by, Opera does the same as before, but the server will also look through what it already has to see whether something is different between the database and the new data. If there is something on the server which was not in the data Opera sent, it will return these items to Opera. Opera will then merge this with its own bookmarks.
When you add, modify, or delete a bookmark, Opera will store this status. Every so often Opera will connect to the server and repeat the synchronization process, this time sending only the changes to the server. Again, if the server has something new for Opera, Opera will get this back.
-Opera Dev Blog
There is no possibility to clear up server stored bookmarks. There is no way to decide that your desktop's order is more important.
I had took a look on this feature more than half year ago. Because of crashes I have downgraded Opera to stable version. Now there is 9.5 "final", so I've decided to try out sync once again BUT it sucks.
During the months of waiting for 9.5 I've managed to categorise all of my links, (make a groups, proper tags, shortcuts) and I've changed my Speed Dial couple times. Thanks to sync tool I can end up with total mess. My desktop PC is always gaining total mess from server and server retrieves additionally duplication of existing bookmarks (but with categories). My Speed Dial is always overwritten and there is no way to prevent that (except copy/paste backup). None of sync checkoxes works properly...
Unfortunately:
Safari doesn't have real extensibility.
Firefox is very slow on my machine after adding dozen of plugins (oops: extensions) to have similar set of features as Opera. Even "Paste and send" requires additional module!
IE - not even worth a comment.
The rest are front-ends for IE (Maxthon) or Gecko (KMeleon) and they inherit pros and cons of others.
Sadly: there is no browser for Windows OS which could satisfy all of my needs (lot of complete solutions and small requirements). I think anyone has not to decide "on what side I have to be" but rather "for whom should I complain"