It is a great low light lens. But it's not optimal for portraits. You would have to be too close to get a decent portrait shot. Generally the rule of thumb is for head and shoulder portraits, a 90mm is the minimum focal length with 135mm being the optimum.
In 35mm terms, the 50mm on a crop body has the angle of view of a 75mm, that's just for portraits, that's the equivalent of a 120mm lens on a 645, a good working portrait lens. I've shot lots of studio portraits with a 35-70mm, and "too close" isn't an issue for me- however if you're looking at fast glass, the only options at f/1.8 in a Nikkor are the 35mm, which is too wide, the 50mm which is good and the 85mm which requires a lot of distance on a crop body (but which I'd rather shoot with when possible.) Unfortunately, Nikon has yet to release an AF-S version of the 85mm f/1.8, so you're looking at ~1700 for the f/1.4 versus ~220 for the 50mm.
I love shooting the occasional portrait with the 400/2.8, but running back and forth to get posing right can be a bit of a pain
Oh yeah, the AF is obviously quieter and faster (at least on my D90) but are you suggesting the D wasn't sharp enough as is? It already blows pretty much any zoom, pro or otherwise out if the water. Since 50mm prunes are pretty much the first additional lens most people buy after the kit rubbish or a cheapo tele, I'm quite convinced we'd be selling even more d3100s in comparison to the pile of rubbish (comparatively) that us the 1100d if the AFS 50 G weren't so overpriced. Is it really worth more than double what a nifty fifty costs? I don't think so! May as well spend the extra dosh and get the 1.4...
The 50mm AF-D lens was 6 elements in 5 groups. The new 50mm f/1.8 AF-S is 7 elements in 6 groups. It's also better coated. The list price difference for the extra element and silent wave motor is $85. The new lens has better resolution at the sides of the frame as well as better bokeh.
Sharpness, is good, 3958 (line widths per picture height) at the center at f/5.6 is as good as it gets in Photozone's Imatest results. By contrast, the 70-200 VRII is at 4028 at 70mm and f/4 and the 24-70 is at 3988 at 24mm and f/4, so indeed it does not "blow pretty-much any zoom, pro or otherwise out of the water." Whilst the zooms do have slightly lower numbers at the far end, they're close enough that you really can't characterize the difference as "blowing" anything away. It's in the same ballpark as the Zeiss Makro Planar f/2 on a full frame body (a bit better actually,) so sharpness is most definitely "good enough" for most uses, but it's definitely not the sharpest knife in the drawer.
I do think the new lens is comparable, if not slightly better than the old f/1.4 AF-D though obviously slower.
With that said, for the price, it's a very good lens- certainly the bokeh improvements over the previous model are worth it to me- I tend to look at lens investments in terms of at least a decade- and I'd gladly pay $8.50 a year for better bokeh in every wide-aperture shot I take. Even at list price, $22/year over the life of the lens works for me- which is why I own one.
I don't shoot a lot of low light, so for me it's more about subject isolation and bokeh than speed- but I don't see great alternatives without spending at least twice as much. I shoot the lens on both FX and DX, and I can crop the heck out of my FX shots without really losing anything- the next option in the line is the 60mm f/2.8 or one of the 85mms.
Paul
