Not much has been said for Dance Dance Revolution S (DDR S), so I think I'll give some impressions here. I signed up for a US account JUST to get this, and in short, it was easily worth it.
I love DDR. The iPhone version is the real deal. It's DDR in true handheld form. Timing is solid, and you only need to hit the correct one at the right time - it doesn't matter f you hit others or are holding down on other pads. There's the proper dance steps, as well as simultaneous steps and holds.
Some of you might think, "What's the point? Isn't it all about DANCING?". Well, true - it is. But even though this is a very faithful version, you still get the feel for DDR. If you felt like it, you can pretend you're dancing by laying the iPhone flat and 'dancing' with two fingers a top. But even if you didn't, when this game is one difficult or extreme, you will be utilising all sorts of crazy twisting tap motions to pull of the sequences.
This is way better than TapTap Revenge and TapTap Revenge 2. With those games, you feel like you are just 'tapping'. DDR makes the sequence and pattern of taps much more engaging and satisfying.
Quite a load of options too. You're able to turn simultaneous taps off, the dancers off, change the arrows, make the field invisible and some others.
There is occasional lag, where the song has a bit of a hitch but I think the rest of it is pulled off well enough it;s forgivable (and I also think there is some deliberate lag in some songs).
Song selection is standard DDR affair - no licensed music that I know of, and a mix of English/Japanese dance/pop tracks.
As a fan of DDR (and having no access to consoles), I think $6.99 is a bargain.
There's a lite version, but unless that has difficult or extreme levels, it may not be a good representative. I never got that, and went straight to the paid version.