I too lost a lot the first week, around ten or 11 pounds. In fact, I haven't lost much since that point, just playing up and down with a few pounds since then.
I do notice, though, when I eat less carbohydrates the pounds seem to melt off easily, so I'm still a "believer."
The less carbs I eat, and the more exercise I do, the slimmer I get. Those two things together are powerful. Alone, they don't do much for me.
Also, I've noticed something important about Splenda...I'm more likely to lose pounds eating a little sugar than eating Splenda products (and believe me I've gone there several times). In fact, Splenda makes me gain so much it HAS to be water retention.
Everyone's body is different, however.