Well, HDR takes longer to process a picture because it's actually taking three. Also, if you turn it on, it saves two copies of the photo, one original one HDR. You can turn that off but sometimes the original is better and sometimes the HDR is better. Really up to you. I find with the 4S I don't need HDR as much as I did on the 3GS.
Well, for one, you can't use the flash with HDR. At least not on the iPhone.
EDIT: It's also much slower shooting because the phone is actually capturing the image three times (at different exposure levels) and merging them into one image. So if you need to shoot for speed, HDR is not the way to go.
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3)
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3)
No. It is not about clearness. It's about exposure. It combines 3 photos, one underexposed, one original, and one overexposed and tries to get the best quality. HDR stands for High Dynamic Range. Here is a decent article: http://www.macworld.com/article/153850/2010/09/hdrontheiphone.html