How well does it work relative to TIFF files?
Well, if you refer to the final size for lossless encoding, remember that TIFF image data can be compressed using various algorithms and the format can be extended, so your mileage may vary. Nonetheless, just for this aspect, it should compare better than the usual TIFF containing LZW compressed data, though (but I'm not sure this is still the most common lossless compression scheme being used for TIFF).
Plus, it behaves nicely on incremental decoding (less bytes to transfer to have a general idea of the whole picture). Adding further image metadata (e.g. EXIF tags) shouldn't be a big problem once the (compressed) image data format is stable.