Many people tell you to just go out and swim lap after lap to build endurance, but TI focuses on minimizing energy output while maintaining forward progress.
To me, that means im fresher for the bike and run, which is nice.
Granted, if you arent exactly new to swimming, there is going to be a learning-curve to "unlearn" some of your old habits, but once you figure out what parts of TI you want to adopt, you will notice immediate results in terms of strokes / length and less energy used.
FWIW, i swam casually my whole life, and before TI i was swimming 25m in about 22 strokes. After Ive figured out their "method" im down to 16 strokes within a similar time.
I read the book and got the jist of it, but watching the demo's on youtube and their dvd's gave me the visual for how to tweak my efforts just that little bit more. Their DVD lessons are online now and are quire a bit cheaper than the entire DVD.