I tried to learn speed reading and got pretty decent at it, I can still do it to an extent. In all honesty, I remembered more about texts that I speed read than texts I would read normally.
The only real difference was that when speed reading I didn't have the feeling that I know the full text, but when tested the knowledge would just magically appear out of nowhere; knowledge I didn't know I had.
Eventually I stopped speed reading because it's too much effort. Usually when I'm reading it's to relax the mind, give it a low effort activity to keep itself busy while I rest. Speed reading seemed counter productive for that sort of thing.
Also, speed reading is rubbish for fiction books. You get none of the pleasant pictures in your head and watching the book like an imaginative movie, but all of the feeling of consuming information at a breakneck pace without much time to take pleasure in it.
PS: my main issue with normal reading is that it isn't taxing enough. Especially when I'm trying to study something. Then my mind starts looking for a distraction and I suddenly realise I have no idea what I'm reading. Or simply get bored of it. Speed reading very effectivelyf orces you to focus on what you're reading (probably why I have a higher retention rate when speed reading)
I'm skeptical of the claim about subvocalization made here as well, but I'm realistic enough to know that I'm not different enough to fall outside the realms of the study referenced.
 My reading speed in this `mode', so to speak, ranges from 300-750 WPM, depending on how engaged I am in the material. I measure comprehension by testing myself via asking others (with a copy of the material in question) to quiz me on the content after such a reading session.
I think another way to test to see how well someone speed reads is to have them pick out a concept out of non-indexed data. If someone speed reads beyond their ability they will skip over it, if they are not a speed reader it will take some time for them to find it.