Hi. We've got a gro-clock and, for us, it's fab. Little Man (3yrs) is often woken up at 5am by his little brother and this has helped him learn that that doesn't necessarily mean it's morning time. It comes with a cute little story as well, reinforcing the message that 'sleep gives you energy'.
I have heard though, that it's overall success depends a lot on your lo's temperment. Little Man is very obedient, so loves the independence his clock gives them. Other's I know have told me that it worked initially for their lo, but eventually they just ignored the clock & got out of bed anyways.
From our personal experience though, it's a thumbs-up for the gro-clock!