It's definitely worse for reading through complete project code. For reading a single-file it's one click more than github which I can live with. It's very nice for getting a quick view on file-based changes, why and when someone did them and I use that interface a lot for that. I find it way easier figuring out file changes there than in github. SF is generally a lot slower (sometimes so horrible slow that browsing is near impossible). GitHub on the other hand sometimes freezes my browser when it has a lot of syntax highlighting to do (not sure if it still does, been a few months since I last run into that).

They have different ways to present code with different design targets and different problems. I prefer GitHub for reading code online without having to check-out, I prefer SF when already working with a project where I have the code locally and using the online interface to hunt for changes.