IMO the hammer is easier to dodge than the ranged poke I mentioned. Jayce is a killer bottom lane early game. The W+Q ranged poke ends up happening to me at the rst timing, usually after my other bottom has home teleported. The Jayces I played (happened a few times now) can set it up even when I'm turret hugging to do W+Q, draw turret aggro while the other laner then runs through portal and attacks me for either a win or a guaranteed return to Nexus. Meanwhile, Jayce switches to melee and tanks 3-4 turret hits to guarantee his lane partner the time needed to kill and/or chase me far away. TL;DR? It basically means the 2v1 ends up in an easy kill. I'm glad to hear it's hard on the mana, at least, and the usefullness dies off early game. Still in duo queue, I can see two teammates doing Jayce constituting the support with his duo partner as a pro ADC to take advantage of these situations. It kinda makes me want to buy Jayce, tbh.
On the other hand, Jayce's melees are easy enough to dodge. If I play bottom lane I like ranged carries like Ashe and Kog'Maw, so as long as I continue to poke the minions and Jayce far enough away, he doesn't even think about using his melee stance.

I can see how the unsuspecting melee carry can end up getting a nasty poke from Jayce, but it doesn't happen to me as a ranged carry. All in all, I hate playing Jayces bottom lane.