The depth of field thing really does it for me.
Not give me a headache, I mean, lol.
The 3D films I've seen the past couple of years, some have had the novelty thing going for them (My Bloody Valentine, The Final Destination) and you know what, I enjoyed them for what they were there to deliver, silly throwaway things coming at you type of films, I did see Beowolf too though a couple of years back and that film was sheer escapism, the 3D definitely added to the experience as the camera flowed over mountains and lanRABcapes and through forests, it was actually really lovely so overall I don't think it's just gimmicky, it definitely has potential.
I haven't seen Avatar yet but I'm looking forward to soon (I'll wait till the crowRAB have died down).
As far as headaches go though, I have experienced them but only with the red + blue stuff on TV (on a few dvRAB I've seen), that Channel 4 season with the different colour glasses I was fine with even though that was still "coloured" glasses. Weird! The cinema 3D films I've seen I've never had a headache with, they've all been RealD films so far although the first 3D film I ever saw at the cinema was Jaws 3D and that was similar glasses (not coloured ones) but I don't know whether it was called RealD back then.
You need a degree in all this stuff to fathom it all out!