I got slark in a single draft and now I'm sort of slark crazed. He is simply amazing as a ganker. So here's a little slark rundown from my perspective (only a few games played, dominated two, lost one):
Strengths: Strong ganker, strong escape, great mid game carry
Weaknesses: Poor stats, extremely item dependent, mana problems, terrible at towers, stealth dependent
Use: Generally I've been putting him in the short lane. You want a ranged support with you ideally, but one game I was solo vs two melee and was able to free farm. So it depends on who you're fighting. Stay in lane, take W skill then Q and E (doesn't matter the order) and it you have a disabler or strong ranged dps you can probably score first blood with a well placed leash. The only issues are going to be if you don't cooridnate the burst damage with the leash the enemy will run away, or if the enemy has disables to disable you once leashed, or sometimes if they have two uber bursty heroes they'll kill you. Generally this won't happen though and you should score a first blood with any good supporter hero like witch dr, lina, crystal maiden, jakiro or even carrys with good disables like spirit breaker or skeleton king or even nyx assassin.
After you hit level 6 you roam and gank when it's advantageous. That means if you are getting free farm in lane don't leave because one hero kill isn't worth leaving a full lane of creeps. If they don't have anyone in the jungle or their lanes are pushed back don't leave cus slark sucks at towers.
That's pretty much it, just try to keep the farm up, gank and roam a lot. In team fights AFTER the initiate (slark sucks at initiating because he's so squishy) I jump in and leash a support, nukers are good like lina etc. Another cool trick is to pop stealth then leash because it won't break it and they can't disable you or burst you if you're in a crowd.
Again slark sucks at fighting at towers and pushing bases. So you need another carry or good pushers on your team. He can carry late but not against turtles. He sucks with other stealthers too, the one game we lost had bounty and slark. An ideal team is going to be another strong carry like drow, viper, shadow fiend, a disabler support, a nuker support and a tank.
Items is where I have a lot of choices. Because his mana regen is so terrible I usually go boots then orchid right off the bat. Orchid is an awesome ganking item too, especially against escapers since the silence prevents escape. But diffusal is also a sweet item, so is sange and yasha etc. I have been going Orchid, diffusal then skull basher -> abyssal or butterfly depending on my evasion needs. Orchid is a nice item to build in pieces as well since those rod parts give you 75% mana regen at attack stats.
So idk, I've seen a lot of guides that say get all sorts of other items first from s&y to mkb to mordigan to eye of skadi for stats. So I'm not sure the best approach, but orchid is working for me.