I've had an Apple TV for a while now and I love it. I use it all the time. And, I'm guessing, with Mountain Lion I'll be using it a lot more. But I've got no reason to upgrade to the latest Apple TV (save the 1080p resolution) since the software updates come around for free.
However, if John Gruber is right - apps may be coming to the Apple TV next week (or at least announced then):
To me, this is what a preliminary WWDC conference schedule would look like if Apple were set to announce anewdeveloper platform, like, say, apps for Apple TV.
I hope he is right. Because I'd love a new TV experience and to get even more usefulness from my Apple TV. But, Apple will likely get me to purchase two new Apple TVs (one for the living room, one for the in-house pub).