Q: I'm thinking of changing the pickups in my Strat. What are the pros and cons and is there anything special I need to keep in mind?
A: You're not giving us much to go on here! Are you unhappy with the overall sound of the pickups or do you just want more versatility? In general, you can trust the manufacturer. They install an appropriate pickup (or sets of pickups) to keep the cost of the instrument at a specific point. If you own a mid-priced electric or even an expensive one, you might try swapping out pickups if, for example, you want more gain for your solos. You may have noticed that Fender has a number of similar Strat models, but with different pickups in some, including a humbucker in the bridge position (the Deluxe Lone Star Strat for instance). Likewise, there are Telecasters with humbuckers in the neck position (i.e. a Vintage Hot Rod '52 Tele). Also, more and more guitars have humbuckers with coil-tapping abilities, which offer a good semblance of the trademark single-coil sound. Our best advice is to choose the guitar you really want right from the start, rather than trying to make a Strat into a Les Paul at a later date.
That said, you might find a killer deal on a particular guitar that plays great, but doesn't really have the perfect sound. This is why third-party manufacturers started marketing alternatives to standard issue pickups. A good example is the Gibson P-94T, a single-coil pickup in a humbucker-size housing. Fender Vintage Noiseless Strat pickups are great if you're doing a lot of session work where noise from overhead lights and computer monitors are inducing a bad hum or buzz from your stock pickups. And yes, on occasion even a really good pickup goes over to the dark side, requiring replacement. If you think of pickups as a fix, you'll probably end up disappointed. It's always better to spend a little more up front to get exactly the sound you're after.