My first taste of "proper" VR was on a mate's Rift DK1. I think that was great because it was the novelty. What I took away from that was the genuine fear of falling off of high things, which I don't get gaming normally. The 2D screen is a safe level of isolation from the world of the game.
Nowadays, I'm playing VR on a budget - an old business i5 machine with a second hand GTX 970 and PSU slapped into it, plus an eBay Rift DK2 which didn't cost a lot. The DK2 has worked with everything I've tried on Steam VR, to give me a feel as to whether to fork out for a "proper" expensive headset.
The thrill of "being in" a game world doesn't wear off. Subnautica and Obduction are such examples. In Obduction there are paths to walk along alongside a mountain with a huge drop to one side. It looks pretty on a monitor, but it's awe-inspiring in VR. The same with standing and looking up at structures that tower above you - much more immersive in VR. Subnautica is just beautiful to swim around.
But, it is a bit tiring on the eyes. The screen door effect is completely annoying - sometimes I can get submerged in the game enough to ignore it but not for long because it's right there in my face. I understand the CV1 and Vive are better in that regard than the DK2, but it's still there to an extent. The technology needs to move on at least another generation to really make it properly viable in my opinion (caveat - I've not tried the CV1 or Vive yet - but I read reviews that grumble to an extent about the screen door effect).
So I play both 2D and VR - the former usually if I am sitting with someone and want to talk and share the experience. VR when I want to feel what the world is like around me.
When I'm playing 2D, I feel like I'm missing out on the immersion.
When I'm playing in VR, I feel like I'm missing out on a nice detailed screen without obvious pixels and that annoying screen door effect.
VR is where gaming will end up, I don't doubt it. I really want it to be good enough to use all the time. For me the DK2 isn't quite up to it, but I still can't resist strapping it on for a couple of hours to get the feel of a place that can't be captured in 2D.
And there is still the huge entertainment factor of having guests over and watching them try to stand up while on a VR rollercoaster. It was worth the expense just for those laughs.
3DTV I don't care for at all. Tried it, it was OK. But VR is something else.