My 2.5 year old ds is still in a grobag. He is a really restless sleeper, so they were a life saver for us. He started in one at 6 weeks and has slept in a heavy or light one ever since (summer/winter). Now that he is old enough to pull his blankets up, we prob wont use one this summer, but there is no way we will stop using one while it is this cold.
Definatly keep it, in the unlikely event you dont use it, the resale on them is huge, so dont take the tags off or wash it until you know for sure.
I luffs our gro bags lol