I love the design of this phone. It is super thin but doesn't feel cheap. The American T-Mobile version of the phone is called Gradiant Blue which has this smooth gradiant change on the back case from a cobalt blue-like color to black. I think it look nice but still personally prefer the international Black version. Still, this is the best design I have seen yet from an Android device and I like it better than the iPhone.
This might be the only downside to this phone. The screen is a nice 4.3 inch AMOLED screen. This means it is on the bigger side (but not too big) of screen sizes yet still retains the vibrant colors and deep black levels you would expect from an AMOLED screen. The only issue is that it is a Pentile display. Please check out the link for the technical explanations for what that means. What it means for me though is a noticeable type of pixilation around the edges of images. Many people don't notice this so your milage may vary. This is one thing you should check before getting this device.
This is the best camera I have ever seen on a phone. Here is a sample photo and video I took with the camera. Unless you are serious about your photos this should your replacement for any point and shoot camera you currently use.
So far the battery life has been great! It easily lasts the whole day on a single charge. Even under heavy use it has done great, which is good since the battery can't be replaced. This is an issue some people have had with the phone. Although this is the way the iPhone has always been, Android phones have usually had user replaceable batteries. Traditionally Android phones have had TERRIBLE battery life and many users even started to carry around an additional battery in case they ran out of power at the wrong time during the day. I never needed to do anything like that with my Samsung Vibrant and so far the battery life on the HTC One S is far, far better.
HTC uses a custom UI for android called Sense. With this phone you get Sense4 which is a very welcome upgrade from past versions. Although Sense has always been better than TouchWiz from Samsung and MotoBlur from Motorola it has never been better than the stock android experience you can get from Google. I think that has changed now and personally i like Sense4 much more than stock Android 4.0.
Many Android users like to be able to "root" and fully customize their phones. The HTC One S is a bit of a mixed bag at this time for full customization. You are able to unlock the bootloader and put on a custom ROM, but due to the lack of true S-Off there are some major limitations to what those custom ROMs can do. Right now popular ROMs like Cyanogenmod and MIUI are still very much a work in progress. This should change in the near future with the full release of the source code from HTC. Until then you are a little limited with the number of ROMs available for the device.
This is the first Android phone that i will honestly admit to being better than the iPhone. You don't have to root it and put on a custom ROM to make it into a decent phone like you do with most other Android phones. Everything is pretty much perfect with the small exception of the display being Pentile. I think this is the phone to get right now and personally think it should handle itself nicely against any upcoming phones.
PROS: Amazing design, great battery life, custom UI is actually good, free Dropbox space, camera
CONS: Pentile display, no S-OFF yet, large amount of bloatware