There's already been discussions about this, but I can't begin to tell you how disappointed I've been with my 4s.
I used a Windows phone a few years back, and since I don't use a ton of apps, figured I would go back to it. Big mistake. I don't mind the lack of apps, but what IS there never seems to work. I'll bullet point my major frustrations.
-Mobile Edge browsing is glitchy and slow, with frequent buffering on videos (even over wi-fi). Just give us Chrome already!
-The camera has a setting to "prevent accidental launch" which does nothing of the sort. I would honestly pay good money to disable the gigantic button on the side of the phone altogether.
-Speaking of buttons. It's like the lock screen sometimes just doesn't feel like locking. I can lock the phone, put it in my pocket, and then hear Cortana yelling at me 20 minutes later...
-Everytime I restart my phone (once a week or so) it deletes all it's data related settings. I have to manually enter APN and MMS settings so things will (kind of) work. I will be honest, I don't even know what those things are, but google told me to do it and it worked.
-Had to warranty the phone in the first couple months because all the speakers went out. Got the new one, and if I use headphones it will randomly disable the speakers even after being removed. The only solution was to hard reset the phone. Didn't bother doing another warranty. I thought the "handling fee" was kind of stupid. I'll just stick with bluetooth.
-You really can't talk to T-Mobile about this phone. No one knows how to use it. The person in store said they'd never used a windows phone and just looked at it like it was going to bite them, and the reps on the phone just tell you to pay the fee and warranty it.
I've had a ton of phones in my 10+ years with T-Mobile and this is the worst one. I'm going to have to bite the bullet, pay off the 50% and jump soon.