I recently tried BannerBoo and compared it with Viewst. I liked BannerBoo more for:

(i) Number and types of templates
(ii) Smart animation feature to create quickly variations for the same template
(iii) Possibility to embed

I actually found Viewst easier to use at the beginning and I think they do a better job in terms of resizing the image for different banners. But overall BannerBoo seems a more mature product. The pluses above are more important for me and for this reason it is the one I decided to keep.

One tip: please give the option to eliminate the promotional banner in the dashboard which is very annoying.

Overall I am satisfied and recommend it.