nah it doesn't affect your typing at all. unless all you do is type in slang, it doesn't predict all the slang words real well. but since it's so fast and accurate, you don't even need to use it.
i like T9 as much as qwerty tbh. just depends on whether i feel like typing with one or two hands lol
you should adjust the speed vs. accuracy settings for your first time. i set mines to 750 for my first time and it helped a whole lot. then turned it down gradually. the swype tips definitely help too.