screen protectors are definitely a good investment. they keep your phone looking good, they're interchangeable and will protect your resale value if you ever want to sell the phone. I had a pair of Power Supply's Crystal screen protectors on my 3g and they were very good. Thanks to the screen protector and a Bodyguardz back cover, I could throw the phone in my pocket with keys, coins, w/e and after a year there were *no* scratches whatsoever. I ended up paying something like $20 shipped for the screen protectors and I've used other ones that are cheaper for other phones that I could swap out more frequently so I'm going that route w/ the 3gs. Just ordered 3 for $3 shipped on eBay and plan on just swapping these cheaper ones out more frequently to keep the screen more vibrant w/ more frequency.
whichever ones you pick, definitely get a screen protector!
also, it should be noted any half-decent screen protectors will not decrease the screen's brightness or clearness to any appreciable extent and will not interfere with the touch screen capabilities of the iPhone.