After Heimers rework, I played him a lot top lane. He's fantastic and an amazing pusher. Get a wave to tower, drop turrets and ult and pew pew pew.
You don't want to play Heimer mid. You have too much ground to cover. Top lane is amazing because you can play against the wall. Also play the bushes. If you are pushing lane, drop two turrets in the push and on mid lane. Bunch them up close enough so they have a good kill zone, but not close enough that one good AOE knocks them all out. You will probably die a lot, but you'll also get a bunch of kills with you get 2v1'd.
Rockets are a trap. Level R>Q>E>W.
Start Barrier/Flash, Dorans Shield. Go into Chalice, Zhonyas, Athenes, Rabs. Run whatever boots are appropriate. I usually do Tabis. Start with at least 10% CDR from runes and masteries.
Heimer is so fucking crazy. I love him, but you can easily counter him. You just have to take out his turrets early, one by one. The turret cooldown is very high early on. Also, buy wards to put in your own brush so you can see the turrets in there (if you're playing vs. him top).