Works okay, some sites are better than others, but I have really poor internet speeds so I don't mind.
My only complaint is that some forum sites that use BBCode tend to have a string of code from textise injected into any posts I make, even if I remove it from the post it gets re-injected as it posts to the board.
I checked and textise.net have done an update, I'll try to fix it in the next update, but you have to know that textise.net adds the next strings to the url: #main, #content, #start_here. The last one is new so I updated the extension so it can be untextise without trouble. I don't manage anything of textise.net so if my addon isn't the problem please write to the developer of textise.net. (please post the url/website with problems)
As the developer of the Textise service, I'd like to say that I fully endorse this add-on. In contrast to my own add-on (which is now legacy software), it's compatible with the latest version of Firefox. This saves me a huge amount of work in trying to keep up with the changes that Mozilla keep making and I urge all previous users of the Textise add-on to switch to Textise-It.