I think I'm in a unique position when it comes to the PS3 slim. I'd say I have mixed feeling about it. On one hand, I'm happy that it's been slimmed down, weighs less and is double as power efficient (an important point this one). On the other, I don't play my PS3 much at all these days and even though I'm a sucker for the latest technology, I don't think it's worth the money and effort to trade for the new slim.
One reason I don't play it much is because I don't have much time these days, I don't play online and the final reason is that I was tired of getting the short end of the stick with multi-platform games. I've got an Xbox 360 too, which I'm quite fond of.
Many games that are available on both systems are almost always better on the 360 than on the PS3. This isn't always the case, but 95% of the time it is. Usually the Xbox 360 version has better graphics, faster loading times, more polished and has no massive GB's install before you can play the game! If the Xbox version is better, I grab it instead. So I only play the PS3 exclusives (the good ones) and my PS3 doesn't get much love. I'm looking forward to Uncharted 2 though, it looks amazing.
If I played the PS3 a lot, I could probably earn my dollars back on the electricity bill, but really, it's practically gathering dust. While I like the slim, there's no reason to upgrade for me, my PS3 works just fine, even if it is power hungry. But if you've been holding out on buying a PS3, there's never been a better time to get one, so grab one now if you don't have one, there's plenty of excellent exclusives to play through.