Absolutely, the compression part of it (LZW) is, yes. (LZW has been out of patent for about a decade, by the way, but it isn't particularly noteworthy on its own.)
GIF looks like crap on some images because it's paletted to [2,4,8,16,32,64,128,256] colours - so many images are reduced to a palette (say, with an octree) and/or dithered (perhaps badly, as dithering tends to increase noise, if not entropy), and also sometimes because some techniques exist (one implementation can be found in Photoshop's "Save for Web") which perform lossy transforms on the data so LZW compresses it better - the result is noisier, however, because it intentionally introduces repeating patterns.
GIF looks like crap on some images because it's paletted to [2,4,8,16,32,64,128,256] colours - so many images are reduced to a palette (say, with an octree) and/or dithered (perhaps badly, as dithering tends to increase noise, if not entropy), and also sometimes because some techniques exist (one implementation can be found in Photoshop's "Save for Web") which perform lossy transforms on the data so LZW compresses it better - the result is noisier, however, because it intentionally introduces repeating patterns.