I got the first Grove case for the iPhone 4 and have had it for about two years now. I am a huge fan. It looks great, feels good in the hand, and the ability to remove the bottom portion to dock the phone in my alarm clock is a really nice feature. I haven't had any issues with splitting, and no degradation of signal while using the case.
I preordered one for my iPhone 5, and figure I'll have it for another two years. If it does crack because of a drop, they have a nice policy of selling you a new one at a discount. However, unless you're prone to dropping your phone, I don't see its relative fragility to be much of an issue. I'd much rather replace a case then the phone itself!
It may not be for everyone, but in my experience it is a high quality product. I'm looking forward to seeing the differences with the new design.