The S4 is the king on specs (i'm thinking that processor will blow the HTC One out of the water). But not the best overall package.
BTW: The S4 is SMALLER than the S3 despite the bigger screen (and lighter )
S4: 136.6 x 69.8 x 7.9 mm (5.38 x 2.75 x 0.31 in)
S3: 136.6 x 70.6 x 8.6 mm (5.38 x 2.78 x 0.34 in)
I have to say that the dimensions and weight are really impressive to me.
It's smaller and lighter than the Xperia Z (same thickness though), but based on the reviews I've read the Xperia Z has an ergonomics problem when held in the hand for a while - the S4 is more impressive when you think that Samsung still included hardware buttons so none of the 5 inch display is lost to the software button bar. Most dimensions are better than the HTC One as well - thinner and lighter with a bigger screen.
The part that flabbergasts me is the larger battery, the insane number of sensors, a full 5 inch display and then an Octacore? How do they fit all of that in a package that's thinner, but about the same size as their previous flagship?
As with everyone else, I'll note that the Xperia Z and HTC One have beautiful designs and the S4 is not visually interesting by comparison, but I think that Super Amoled screen and the tech inside give it the edge (for me). Looking at a new phone the Xperia Z reviews all note the poorer screen quality, poor battery life and not so great ergonomics. The HTC One seems much better than the Z, but for me the limited storage (according to their Facebook page they will only launch the 32gb version in SA) is a deal breaker, so however good the HTC One may be I have to stop looking at it right there (though I bet the storage probably won't be an issue for 90% of the people out there - in which case it's a much harder decision).
Overall I'm impressed with it, if I owned an S3 I'd see no reason to get it and I've noted before the Samsung strategy is some improvements over previous models (but nothing earth shattering) so that an S2 owner ready for an upgrade now will be more than pleased with an S4, while an S3 owner won't feel like they lost out by not waiting for the S4 to be released. I have a Galaxy Note 1 and the considerably smaller footprint, thinness and lower weight with only a slight loss in screen size makes this the perfect phone for me now. I'm in no way a fanboy of Samsung, in fact, I really wanted to try another manufacturer with this upgrade (and I think the HTC One is a solid choice for those who don't need the storage or removable battery), I'll certainly wait for the reviews to come in, but it looks like the S4 is a winner to me.
PS: I generally like Touchwiz and after trying all the major launchers in the Play Store I always come back to Touchwiz. It's really improved with every update to my Note.
PPS: Why are some people complaining about the pentile screen? Seriously, you use your phone an inch away from your face? That's about the only way you'll be able to tell the difference in reality, or maybe you walk around with a microscope which you pull out before you use your phone? At this pixel density it really makes no difference.