It's worth it.
I'm going to give my full review, so please excuse the fluff, but I think its relevent in the new 'One' world.
I bought one because I wasn't happy with Sprint in my area. I really wanted the One X because of igher resolution screen, but I wasn't happy with hearing this is only under AT&T.
The One S is smaller, it is actually testing faster (benchmark) than the One X. The Sprint Eco LTE is very nice, but it's like buying a Corvett without gas! A wonderful phone withought the network to back it up. Unless you're in a Sprint LTE region, you'll be left wanting more than Sprints .38mbps 3G (supposed to be).
AT&T is a great network, it I have personal reasons for not liking them. If its the One X you want go with AT&T.
If its a fast network at a good price, go TMobile (as long as you have coverage in your area).
The reasons. T-Mobile will only work fully with the One S because of what's onside. The HSPA42 is fast. I get between 10-15 mbps in my house, 10mbps at work and average 7mbps in travel. Compare this to others (I've only had Sprint so I don't know how AT&T weighs in terms of speed).
If you flash the One X to T-Mobile, you're going to get super slow Edge. Mayne prepay is the only logical option for flashed users.
I was hesitant in buying the One S because, it has a dif processor, smaller low res screen, lower quality face cam and not on LTE. I decided to give it a try and I'm amazed.
I get free tethering, 5GB 4G then unlited throttled speed & unlimited minutes, text, WiFi calling all for $115 month. I've done benchmark testing and have come up ahead (by far). I'm faster than the Sam S2, Note and others (Even One X). The screen is super vivid and bright. The camera is worthy, the speed of the processing is on point and ill agree that the size is on point too.
Cons: this little sucker gets warm on 4G. The front camera leaves you wanting, the screen isn't the absolute best
Pros: lightweight, fast, great speed of network, decent battery and very responsive.
Middle of the road: SD card (none)
In a nutshell: this phone is a powerful, fast and most up to date handset out. For the money It's worth it.