I have my first GWT project that I created using UiBinder (GWT 2.0 way) which I found to be easier than write my UI creation Java source code (GWT 1.0 way).
But I saw this thing called GWT Designer that Google are releasing for free. It has nice features and wizards which were missing with the standard Google Eclipse Plugin. I like it, but I still think that using UiBinder is better. I think GWT Designer will be really useful when it can help you write UiBinder XML files (GWT 2.0 way), and not just source code (GWT 1.0 way).
What do you think about it? Do I need to migrate to GWT Designer project? Will it be better if I migrate but still keep UiBinder UI creation?
UiBinder can give you better performance and a better optimized download than traditional widget construction; to me that's enough of a reason to stick with UiBinder. If your app is light and fast enough as it is then the choice probably comes down to what style of development suits you best.
I wouldn't go as far as to port your existing UiBinder templates into the designer. Google will be adding support for them to GWT designer soon enough.