I just got the Google bumper today. I don't think it's worth the money. Actually, it depends on what you're going for. If you're the type of person who generally doesn't use a case (and therefore isn't concerned about protection from drops), then the bumper works great as a means of keeping the glass off the table when you set it down in order to prevent scratches and sliding.
However, I'm not confident that it would provide any protection from a drop. I've been using a cheap iBlason case up until now and it probably provides better protection than the official Google bumper. But I hate how the iBlason case makes the phone look and feel cheap.
I really wish Speck would make a case for the Nexus 4 since their cases look great, provide a decent level of protection and aren't too bulky... but they've said they won't be making any cases for the Nexus 4.