The lossless mode is great (but more limited than PNG), the lossy mode sucks though. Like it only supports a quarter of the colour resolution compared to formats like JPEG.

Also being a video format, it's not actually tuned to store still images, it likes to blur/smear things.

Edit: But if you're using it for the intended purpose, low resolution previews, thumbnails and stuff like branding, it's fine. I wouldn't use it where quality matters.