prediction tools are based on number of backlinks and the links from high PR. I do think such tools are useful. The reason tools sometimes fail to predict is because links from PR may be dropped during the PageRank update. Or your link partner may not keep your link for some reason. There are many reasons that your backlink quality and quantity may be dropped at any time. And the tool simply can not calculate that. It doesn't calculate any possible penalty either. But its mathematical approach is probably more accurate than we think. So when you are using it, think the tool gives you a prediction based on your total backlinks and quality of them + if you could keep those links. iwebtool probably is one of the best IMO. You should check their site.
the tools just take a guess. No one really knows how G calculates PR. if you try 5 PR tools, they are probably going to give you 5 different answers. Not worth worrying over
Also you can use this Page Strengh Free Online Tool. It uses other factors, not only of number of backlinks.
Use them by all means, but please don't try to sell links based on predicted PR -- that drives me nuts!
I like Page Strength a lot more over all other PageRank prediction tools.. I think its far more reliable and relevant when you are buying links.