Been enjoying the free week with Heimerdinger. Think he might be next on the purchase list for me. Once he gets up and rolling with lvl 5 turrets, he's a defensive beast. I can single-handedly seal off a lane. Had a match the other night where I had 4 enemy champs massed to counterpush bot lane as we closed in on their fort. Two turrets up, W on a quick cooldown, and ult at the ready. My entire team managed to escape the teamfight, and nobody could pursue because nobody wanted to be the one to initiate running that gauntlet. Once or twice a couple of them would start to rush me, I'd ult the turrets for slow effect, W rocket them for hvy damage, and maybe blind grenade as well. Never did get kills, but I'd hurt people enough to drive them back.
I think one of the things that has made him work so well for me compared to earlier attempts is Athene's Unholy Grail. Just a much better mana supply/AP booster for his kit than Archangel/Manamune. He doesn't cast often enough for Archangel to be effective, and doesn't autoattack enough (or need the AD) for Manamune.
Typically combine that with a Rylai's so the turrets have an inherent slow even without the ult, and then add Sorcerer Boots and either an Abyssal Scepter or Voidstaff if the enemy starts building MR.
I won't have a ton of champ kills, but I do well in the assist dept, farm like there's no tomorrow, and most importantly I bring great map control. Only real weakness is gap closers. If a Jax or Xin Zhao gets right up on me (and isn't already hurt bad), I'm toast. No CC, no escape.