Go for it. It's good practice and makes speaking and even socializing less stressful. It feels artificial, but at least they are positive and constructive and very very forgiving.
Pre-written, well, you have a program of 10 speeches you must prepare. They're not written in the sense of written out, but you may have note cards. You won't be encouraged to read from a sheet.
If you are in any kind of a big city, there will be many Toastmaster groups. Choose one that matches some interest of yours. They can be relating to business, sports, drama, environmental, young, old, second language, you name it. If you don't like the first one, maybe try another. They can be very different from each other. Google Toastmasters International and you should be able to navigate to a list of the groups in your area.
I didn't finish all my speeches, but that's because I moved out of town.
It was all right. It was useful. After the basic programs, there are different choices such as comedy, persuasion, technical presentations, et cetera, that you can specialize in if you really like or need practice in one area.