From what I've been told (it's just one person's opinion)... unless you want to string professionally, it's really not worth anything to anyone.
After stringing rackets for a handful of years and being around shops for a few more... I'd vouch for that. No one really cares.
That said... I think the RSI magazine is a great read. I think the stringing instruction books are essentially worthless. I think the cost is too high, but the information to study for the test (and mrt test too) is quite good to study regardless. As for the free strings... that's nice and all, but I'd rather experiment with what I want to and use that membership money to buy string. I haven't gotten to use them... but one thing I'm jealous of is the USRSA online calculators for various racket specs and whatnot.
Babolat Pure Control Tour | Poly ~58lb | USPTA