For me, the combined mode is pretty underwhelming. It's big. It's heavy. It's one big brick that doesn't hold together all that well. It's kibble-tastic with a huge heavy backpack that puts a lot of junk in the trunk.
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I have Jetfire, but I got the add on kit for it, because it looked better to me. Though the kit is poseable, it's not made with the most sturdy plastic, just above a model kit, and would not survive a tumble off a shelf intact. Not to mention it was expensive.
Bottom line, if you got things for cheap, it might be OK for you, but do not pay bank for it.
Otherwise, with the studio series coming out with Jetfire, you might get lucky and they might build in a combining gimmick for it to combine with an existing prime or one coming out in the future. But that's just pure speculation on my part.
