I feel that it's pretty much a must to have one these days. I have the D-Tek Smarty BX1500 in one of my vehicles and I think it's time to upgrade it as well. I'm not familiar with the new models these days, but fortunately there seems to be many options, back in 2010 when I got the Smarty is was the best and pretty much the only decent option.
What you need to look for is one that records at least 1080p HD at 30 frames per second, minimum. The BX1500 I have only does 15fps at 1080p and it sucks, because it misses something that happens fast. And we know how fast accidents can happen, if someone walks in front of you and you need to proof in court that you had no time to avoid running him over then you will be happy that you paid more for a high fps camera.
You also need one that can loop over videos, but I believe that pretty much any dash cam will do this anyway. Lastly you want one that uses a large power capasitor, so that it can still record of a few seconds after loosing power, this is important so that the camera doesn't corrupt the video when power is cut in the case of a large accident.