If you want to learn to ride a bike properly off road, then those in the know recommend using flats while you are learning the skills, (you do pick up some bad habits bunny hoping and jumping with clipless pedals), and once you have mastered the skills then move to clipless if you want, however, it all depends what you want/plan to do, I've always used SPDs, (I can live with my bad habits) I have XTRs on my hardtail and full suss, I think they're great, and the release is so natural I very rarely get clipless moments, I ride quite a lot of techy black stuff/natural trails, I find all accidents I have are nothing to do with be clipped in, me and bike are always separated once the crash has finished.
As several have mentioned it really is a personal choice, I was out on Saturday, beautiful natural trails in the Scottish Borders, I was clipped in, mate was on flats, we rode the same stuff no problems, he was faster on the techy stuff, but that's 'cause he's a better rider, I would like to, but I can't blame it on my pedals.
