Harry potter, what can I say, I collected the complete series, and was offered quite a bit to sell it, I never did like most of the sets Lego produced on this, as I thought it let the films down. It only started to get interesting when in the last to years, when it meant the themes where getting darker, but that too was a let down as they only covered the basics and not the main story.
Still with the new Harry Potter things have gotten better and the sets and figures are vastly improved, and yes, I'll be getting what I can of them and hopefully others I've spoken too will as well. As for going on sale, from time to time yes I have seen it, but you must remember it's gotten more popular since it first came out and although if Ebay and that are anything to go by some collectors haven't forgotten this.
My adivce is don't leave it too long before deciding to buy these.