Slingbox, AppleTV and Netflix are all great ways to improve your home media experience, but all offer different services.
AppleTV does not and will not support Netflix streaming, as they are direct competitors. There are rumors that Netflix might possible offer streaming through the xBox 360. If this does happen, then perhaps Apple will be forced to offer a competing service. Right now, it is pricey, but they've got an easy, great interface for renting movies. I personally find it as an "add-on" to Netflix. I might rent one to two movies at most per month on ATV, for times when I'm in the mood for a particular program. It can of course stream your own material from iTunes, which works great. Of course, it also offers the ability to stream your iTunes music selection and features a nice screen saver. Throw in the ability to view your photos on your TV and stream YouTube.
I don't think there is one killer feature about AppleTV, but rather the overall package. Plus, if you don't have a Blu Ray player, it's a good low cost entry to HD rentals.
Slingbox is great if you:
a. Travel
b. Want access to your TV from any laptop/PC/Mac
c. Enjoy the uber geekyness of controlling your TV from virtually anywhere
For me, I was away during the Soprano's finale. I knew that if I travelled the following day that I would most certainly find out if Tony was "whacked". Logged into my Slingbox, accessed my TiVo and bada bing.
Sorry for the long winded answer. Before buying ATV, I questioned where it might fit in my home entertainment system. It's not a world beater, but I feel it's worth it for $200.