I confess to being another HM40 fan - cheap, rugged, and for my money more comfortable and clearer than DC's 2-3 times the price.
Just for a bit of interest, let's turn this around. There are lots of headsets around, and most are pretty good - the posts above clearly show that the main issues are cost and personal preference, not whether a headset is actually suitable or not.
So, what headsets would anybody specifically NOT recommend for PPL type flying?
For example, I happen to own a Telex 750, which is an ultra-lightweight headset aimed at airline cockpits. I wear it occasionally in a light aircraft - generally where the cockpit is reasonably quiet and I want to be able to hear and chat to my passengers without them having to wear electric hats themselves. BUT, for long trips the cockpit noise becomes quite annoying (the T750 has no significant noise attenuation); for similar reasons I wouldn't wear it if I really need to concentrate on a particular flight (doing test flying of a homebuilt for example). For those reasons I'd specifically not recommend the T750 for the majority of PPLs or students.
G