Many people lose their tempers merely from seeing you keep yours - Frank Moore Colby
The $90 chip will allow you to bring a PSP back from the dead after an unfinished firmware upgrade, or a flash editing mishap, will allow you to switch to all firmwares of the PSP on-the-fly. It will also allow the homebrew launching for those PSPs which do not have the capability.
However, the site says that installation is a pain in the butt, requiring soldering and other messy things.
Installing it will auto-void your warranty and means that you can't call Sony for a replacement if anything goes wrong. In some parts of the world they may be illegal.
Sony might start getting shirty against developers who support the chip and will almost certainly take legal action against the chip soon, the site says. It will also start releasing firmware updates that will include 'security' updates that will tigger the chip anyway.
All up it is probably better to spend the $90 on a used GameBoy Advance, and pop in a few cheap games and is less likely to tigger up your system. ยต