I traded in my original PSP for a PSP slim yesterday. My initial reaction is actually one of disappointment - it really doesn't feel like a trade "up" at all. The molding of the casing looks and feels much cheaper, there is no eject action on the UMD drive bay, the L and R shoulder buttons are wobbly, and rather than a matt finish in the back of the unit it has the "piano black" finish which gets fingerprints all over it and, I'll wager, probably a little slippery after a while gaming. Initial impressions are that they have cut a lot of cost from the unit but not passed too much of that on to the end user (I guess they want to make some profits on the hardware seeing as how easy it is to hack).
I haven't spent too much time with it just yet, but after a few flights coming up I'll hopefully be able to gauge the battery life a little better. I also want to see if there is any difference from the UMD cache in some titles. TV-Out I've not tested yet as it doesn't come with the lead (in fact this model doesn't come with a mem stick or headphones even).
One element that is immediately and noticably different when playing it is the positioning of the onboard speakers is far, far better. Rather than placing them on the bottom of the unit these are now positioned at the front, at either side of the screen just above the control areas, so they are much more audible for the volume range they have and won't get obscured by your hands at all.