Have upgraded my firmware on my N91 to 2.00.052 using the Nokia Software Updater.
So far all is running extremely smoothly only exception being that "Best Taskman" no longer works. An email has been sent to smartphoneware to see if they have a solution.
I have read somewhere that a few people are having issues with certain software no longer working that used to run fine on earlier firmware. Otherwise the phone is rock solid stable now and the battery life is noticably better.
I noticed that Best taskman etc where never explictly stated as compatible with the N91. I was told that these companies that test their apps as "s60 3rd edition ok" test it on one or two devices and not all. N91 has a HDD, LCD that is different from N80, N93, etc so my guess is that it worked before by luck and now it doesnt.