I've wanted one of these things forever and I was given the perfect excuse to pick one up last week when the screen on my GBA died. I was nearing the end of Mario & Luigi: Superstar Saga, when the picture suddenly faded from the screen. I tried changing the batteries to no avail. It was pretty clear that the old launch model GBA had had it.
The GBP arrived on Friday in great condition, apart from a few scratches. Installing it was very simple, I just needed to take off the cover of the high speed port on the bottom of the Cube, slot the GBP in there and screw it into the console. The two screws are fixed to the GBP so you can't loose them. Of course, the unit itself is no good without the required software, which came on a pretty scratched-up GC disc. I popped it in and booted the Cube, hoping for the best. Thankfully, it all worked fine.
I was really impressed with how well the handheld games translate to TV screen, even old games from the original Gameboy look nice and sharp. The GBP presents the games in the middle of the screen, surrounded by a border in the style of a GBA or you can stretch the picture to fill about 2/3 of the TV screen. The second option leaves you with a smaller border and doesn't really impact on the picture quality of the game too much.
So far, I'm really happy with the GBP and I'm hoping it'll get me to check out some more GB/GBC/GBA games. Even though I've had a GBA for years, I've pretty much ignored it for most of that time. It's only within the last few years I've started to pick a few games for it, but given the fact that I just bought Donkey Kong '94 on eBay, I think the GBP may be working.