I just got a NY Lock and wondering if people had any experience with the key registration systems and/or getting new keys from them. Any reason NOT to register the keys?

I moved and after things got sorted out my key was gone. I didn't register, but I had the paper with the code on it. I think I went online and gave tem the #, credit card and got it (2 keys!) shortly thereafter.

Don't you need to register in order to get the bike replacement guarantee?

That's a separate registration process. The key registration is only registering your key number in case you lose your keys.