That entirely depends on your expectations. It can be a good way to get to know yourself, try out a few things, get you to think about yourself and your yearnings but of course, as you say yourself, it can never be the same as face to face.
And then there are many different forms of 'online': Chat and text only, skype, with or without cam. I guess it's a big difference whether you just chat with someone or whether you actually talk to them via webcam.
I'm doing some online D/s with a Mistress, but that's mostly roleplays and writing stories. Still, if I'm in the right frame of mind, it can get quite intense, but it certainly can never replace the real thing with all the sensations.