It is NOT worth 40$/month, because all info they are collecting is absolutely free, so why do you want to pay for it? Basically you are paying to see your PS, which has nothing to do with real website strength. Of course this is just my perspective maybe this is wonderful tool.
LOL! In all honestly, the PS tool is probably one of the tools I use the least, the main things I love about the prem membership is the premium content, and guides which are all a great read and the keyword difficulty tool, saves me a vast amount of time and I learn the lastest concepts in SEM. The best thing (about all the tools) is it saves you time, the information collected is displayed in a nice little package/report, sure that information is free, but being a busy webmaster, I really dont have to time and nor do I want to research everything manually. Sometimes its just nice to get an overview. oh, and a new tool has been released today 'Juicy Link Finder' having a play with it now EDIT: Also forgot you can raise mew questions and get them answered my the seomoz team, anf then theres YouMoz