It's great for simple banner making and the animation options make it pretty easy to add some motion to your banners.
The first time I used it was on a paid job and I used it to optimise GIF file size by uploading an animated video and exporting it out through the GIF option which was extremely easy to do with BannerBoo as it had a lot of export options for gifs, it paid for itself with that one job so no complaints there.
The main reason it's a 4 taco rather than 5 is that there is no batch processing and the API is kept behind a paywall separate from any deals on here. This makes it extremely slow to do working at scale and means you can't automate production. If that was available that would really differentiate this product.