I've tried loads of these, Instapaper, Evernote, Readability, Read it Later, etc. My current favorite is
Pocket (formerly Read it Later) but used to use Evernote and Instapaper a lot.
Until the likes of Google Chrome is available on iOS and all versions of Android, not just 4.0, these bookmark services will continue to exist.
The other problem Google has is App Support. If your reading a news story on Flipboard or Pulse you can save it to your favorite bookmark service as the developer has utilized the bookmark API. Google will need a better API to allow these apps to bookmark stories too to your Google Account. I can't see the likes of Opera ever having an API like that. They also don't have the penetration of Google.
If your an Android 4.0+ user then you can use Google Chrome. Otherwise I suggest pocket, particularly iOS users although I expect Apple to bring out an improved iCloud based "Reading List" as well later this year in iOS 6.
I think all these services have a lot of development to come and this area of cloud based bookmarks is only getting started.