Is the iPhone 3GS still worth it?

I need to make a decision fairly quickly, so I'll be brief. I have roughly $200 to spend on a new phone. It will have to be unlocked so I can put it on T-Mobile's prepaid plan. (I know, it sounds awful, but you get unlimited voice, messaging and data for $2 per day, and you only have to pay on the days you use their cell service - more often than not, I'll just be at home using wi-fi.)

In the past, I've owned two iPhones, two iPods touch, a Palm Pre, and (in the very distant past) a handful of PDAs. I currently own an iPad 2 and a Mac mini, so I'm pretty locked into the Apple ecosystem and loving it. But with my very tight budget, I can't afford a brand new unlocked iPhone. The newest I can buy is an iPhone 3GS. Is this still worth it, or should I give up and buy a Droid or something?