Again, a very personal choice. When I first started looking at the Model S, the multi-coat red hadn't been announced yet. I loved the entire concept of the car, but none of the available colors wowed me (the Signature Red was nice, but they'd all been spoken for and would have been out of my $ range anyway). I was leaning towards the dark grey as the color I figured I could live with when Tesla announced the multi-coat red. I fell in love instantly. Even though the Model S is a big car, that particular red just "works." I placed my order, even though it mean additional agonizing months of waiting since the red wouldn't be shipping for a while (and they decided to charge an additional $1,500 for the color, which annoyed me a bit).
I've had my car since May of last year and I, personally, haven't regretted my color choice for a moment. It truly does look different depending on whether you're looking at it under the bright fluorescents of a garage, in direct sunlight, in shade, even in the rain. It's like having a different car depending on the weather and there have been multiple times where I've just stopped and looked at the color, amazed at how different it can look.
I work at a relatively small company and have definitely the "flashiest" car in the parking lot, and everyone knows who owns the Tesla. But, more than anything, the reaction has been overwhelmingly positive. In fact, I've been shocked at the number of co-workers who have congratulated me on getting it - the general sense I've gotten is that they don't have a problem with it because they know I've been here a long time, I've worked my way up, and have worked hard for it (at least, that's what they say to my face ;-) ). This was an upgrade from a Honda CR-V (and a VW New Beetle before that), so throwing my money around isn't exactly something I'd been known for.
Long story short, get the color you want and be happy. I still marvel in my Model S every single day and I haven't experienced a moment's regret. It's a marvelous machine and a glimpse of the future. My advice, FWIW: buy it, enjoy it, and try not to think about what others think.