I tried using one for a while. As everyone says, I pretty much doubled how long I could make weed last.
Downside: The high is more subtle compared to a joint(/spliff) which has a more noticable "hit". I also noticed that instead of smoking a joint with tobacco, I would vape and immediately smoke a cigarette afterwards to get that unhealthy feeling in my lungs I've been craving. I barely smoke pure tobacco anymore so it might be different for me now but just as I noticed with hash in a pipe, I find the sensation of smoking a joint to be enjoyable somewhat more than the effects it seems.