I recently upgraded from my BB Curve to a 4S, it's seriously a life-changing event.
I also was on Sprint. I called and spoke with someone in Account Management (Retention) and basically made a case on why I should upgrade now (I was grandfathered into the old data plans, you'll have to pay a $10 per month fee PER LINE you upgrade using a Smartphone - it's called their Premium Data Charge)...
They pushed my upgrade up from February 1st to January 15th for both of our lines.
That being said, because of the changes to the discounts and contracts, I would seriously just consider going to AT&T if you wanted to get a 4S. Why?
1. LTE is right around the corner. It's highly speculated that the iPhone 5 will have LTE support. Remember, you're going to want this phone to carry you at least 2 years until your next eligible upgrade.
2. Sprint's 3G data speed is in the toilet, usually at or less than 1 Mbps. AT&T runs on HSPA+ for their 3G network - which they advertise as 4G, but it's not really a true 4G technology - which usually runs between 5 Mbps to 10 Mbps. So you're looking at webpages, video, pictures about 5 to 10 times faster. That's huge.
3. Sprint is in the process of upgrading to 4G LTE, but this isn't scheduled to be done until the END of 2013. That's also assuming they stay on pace. So you could technically upgrade now if you were hellbent on staying with Sprint and they let you upgrade in 22 months, so you'd be eligible to upgrade on January 1, 2014. Or if you're able to secure an earlier upgrade as mentioned above via Retention, you could upgrade today/this week and it would make you eligible December 1, 2013 (because the upgrade is available in the 22nd CALENDAR month)... When this rolls around, LTE should be done and usable. Remember, you can't get 4G coverage unless your carrier provides it. So if you "waited" for an LTE iPhone and Sprint doesn't roll it out until next year, you're 3G that whole time.
4. AT&T's iPhones tend to resell for more money. When you upgrade to the iPhone 6, for example, you can still resell the iPhone 4S to cover most, if not all, of your personal upgrade cost.
Again, I would truly consider AT&T if you can hang within the data caps.
I like the idea of the 5GB+Tethering plan, and might actually have considered that, but I'm now trapped with Sprint for the next 23 months. But who's counting?
For me, they made the offer pretty sweet: They gave me a $100 credit towards the difference in my bill and my activation fees ($36 per line to upgrade), and let me upgrade that much earlier. I just didn't want to deal with the headache of switching at the time.
I think the service is fine, even data speeds, 99% of the time. I really haven't noticed too many issues. At home, I'm at Wifi, so that's not a factor. It's more of a "what could be" issue, which won't happen very often.