With the highly anticipated next generation iPhone—iPhone 5—only a couple more months away, now is a pretty silly time to throw down a few hundred bucks for Apple's smartphone. No sense in buying something that's about to be outmoded; if yours was recently broken or snatched, use a burner and wait it out.
Then again... The 4S is still a great phone: fast, tricked out, slim and sleek. A perfectly good phone, not exactly in obvious need of an upgrade. Now, in anticipation of the forthcoming iPhone 5, Sprint is discounting its 4S by $50 across the board—regardless the storage capacity or color—and they're waiving the $36 activation fee, too.
If this is appealing to you—if you aren't currently in possession of a 4S and want to be now—think long and hard about how you'll feel come October. It all comes down to whether you'll be jealous seeing everyone with the latest toy, or if the money you'll save now is of greater importance.