I think the 4S is an underwhelming release overall as a tech fan and very much so if you have an iPhone 4. But I think for those of us, with 3GSes, it's a very good upgrade. Since we have a pretty good thing going with our Family Plan and being grandfathered into our Unlimited data plans, we aren't going to go switch carriers anytime soon. That being said, I had been looking into some Android devices a few months back to see what is out there. The best one out there seems to be the Samsung Galaxy S II. It has a few specs that are better than the 4S, but many of them are right on par now. Apple essentially caught up with it and a couple other devices, rather than leaping them like they normally do. I think that's where the disappointment stems from. People are used to Apple completely jumping the competition or redefining the market and with this release they've merely caught up, at least hardware-wise.
For me, the biggest issue is that my headphone jack has not been working since around Jan/Feb. I can't listen to anything with headphones which is extremely frustrating, so I want to get a new phone. I waited specifically for the announcement of this one to see what Apple would do. While the Galaxy S II looks great, the only major thing that would really convince me to switch is the fact that it can do 4G. The problem is that 4G is still not very widespread and from what I've read, most of the time you end up getting kicked off onto 3G. Verizon's LTE is supposed to be a bit better, but as I already said, we aren't switching networks so it's a moot point.
I'm very much tied into the Apple ecosystem now with the iPad 2 and my Macbook Pro and iOS 5 looks really great. Android still can't interact with the rest of my Apple products quite as well so the cost of switching to a new OS is hard for me to justify. Since most of the specs of the 4S are on par with the Galaxy II, the only real big reason to switch is the 4G, which I've already mentioned.
The 4S will certainly be faster than what I have with the new processor. It also has the better screen that the 4 has. And the cameras are clearly way better. I don't even have a point-&-shoot camera so this would end up being my camera. The Siri thing is nice to have but isn't necessarily something that I need. I know that there are a few features in iOS 5 that won't be available on the 3GS but I can't remember what they are. The integration with iCloud will also be nice as well.
I'll read some more thoughts and reviews on it before making a decision, but will probably get one. From the comments I've read so far the consensus seems to be that no one with an iPhone 4 should get it but people with a 3G/3GS should. I'm also going to take a look at the Nexus Prime, which should be announced next week. I simply am tired of not being able to use my headphones with it when I have all my music on it so I need a new phone soon regardless.