I got wonder weeks after a recommendation, it's based on leaps not weeks. It's okay but I'd say if you're someone whose hung up on milestones or may panic baby isn't doing something/you're not doing rlenough then don't. If you're happy with a guide on development and some play ideas to support then do
I'm using BabyCentre, it also has a lot of useful articles
Baby+ has articles, milestones and says how they're developing week on week
I have wonderweeks which is ok but I don't think it's amazing so would be interested too