I've not done many yet but from what I have done some are smaller some very similar to a png in filesize. You do get the advantage that if the user zooms in on the page your svg's will still look very crisp! Plus using base64 encoding you can embed the graphics into your web-page making less http requests and more speed for the user / less load on your server etc.

I am no expert, but they seem like a good option if you are producing content for retina displays etc.