The problem is that it requires me to navigate away from the page I am currently on...I don't think one should have to do that constantly just to check to see if a post had been made if its not really necessary. Why not just constantly have to click the PMs link to check to see if you got a new one? Would that make anything better...no, no it won't.
Any software/web designer can tell you, the fewer clicks required to achieve any thing, the better.
And as for the script itself...it doesn't have the functionality I was hoping for. It informs me to any new reply to ALL threads I've ever posted in, not ones I've bookmarked specifically. And while, yes I can tell it to 'ignore' replies in said threads, they are still listed in that particular tab, so even if it says I have new posts to read in topics I haven't designated to ignore I have to actually search for the ones I consider relevant. That's not as useful as simply notifying unread posts in bookmarked threads.