For me, I wouldn't consider buying an iphone4s at this juncture, not when for a little more, I get a substantially better 5s which is also assured of more upgrades. Much less be locked into it for 2 years.
For one, 8gb is woefully inadequate for my needs.
That said, if I were given a 4s to use (say handed down by a parent who upgrades), I would be pretty contended using it, in that it still lets me access most of the features accessible to IOS.
For one, I find that IOS7 still runs quite smoothly on my 4s (which I am still hanging on to even after upgrading to a 5s, I just can't bear to sell it). Its dual-core processor allows it to multitask and handle graphics fairly well, it sports Siri (which Apple is starting to really put an emphasis on - it's pretty accurate these days, and I find myself using it more and more). It still has a 8mp camera. I still get the full library of IOS apps, and most will run smoothly on it. Plus all the goodies of the Apple ecosystem, like 3g FaceTime, turn by turn for maps etc.
I lose 4g, apps will load a tad slower, but nothing I can't live without. In that context, yes, the 4s is still a pretty solid smartphone even by today's standards, but I wouldn't spend my money on one.