As long as you understand you may have to jump through some hoops to upgrade firmware, and don't rush to upgrade each time a software patch comes from Apple so there's time to get the hacks upgraded and tested, there aren't any major downsides. If you do ever have issues, you may need to revert to a standard Apple state to get warranty service, but it seems that hasn't been an issue for people who heed warnings about not upgrading firmware blindly.
Personally, I'm waiting to see what the SDK looks like and what people are allowed to do with it. We're in the very early days of iPhone programming and the platform, and it doesn't seem to me like anyone has done anything exceptionally compelling (though some things are certainly very impressive and appealing, like NES emulation, for instance, but on the whole it seems like there are more proofs of concept than useful apps). What are you planning on installing?