Google has recently developed a new Google Docs version and if you thought they took the chance to improve (at least a little) the compatibility with other browsers, you couldn't be more mistaken. Actually, it went worse with Opera.
If previously it was enough by adding ?browserok=true in order to enter Google Docs with Opera and mask it as Internet Explorer to use Spreadsheets, now it gets constantly reloading with Google Docs and Spreadsheets is only usable with a poor read-only interface (masking Opera as IE throws errors).
The first problem has an easy workaround: it's enough by adding a new string in the URL which must be different every time the browser is started. I don't know a solution for the second issue yet.
And in order to make it easier for Opera and Google Docs users, I modified João Eiras' script that patches Google services making them Opera friendly to add this workaround. By installing this user script in Opera you will be able to use Google Docs and other Google services with Opera.