I'm not very impressed with my 1-4. It's great in terms of portability and convenence and all that, but just isnt that sharp. I chose it over anything else because of the IS. I've often wondered if I got it calibrated if it would be better, but frankly, I'm not sure I want to spend another 100 on the lens.
Also, it's focus is significantly slower than other lenses I have; that will be a huge factor in bird shots. Going between it and a 70-200 when shooting sports is painful in terms of AF.
The day they make a 5.6 IS, I sell the 1-4.