Riki's pretty strong yo. He's like Ursa where if you get a good start you can roll all over the enemy. His backstab does crazy amounts of damage and the AOE silence + blind is strong as shit.
Of course, unlike Ursa... he can't actually jungle... or kill Rosh by himself... or control a lane incredibly well... or lane at all actually...
Okay, he's bad at jungling and killing Roshan alone, but he's not a bad laner by any measure. Backstab lets him deny creeps like crazy, smoke is incredibly good at picking people off with some help. (That silence is really long, and has a slow!)
My group likes to do the classic Riki+Wisp combo, where Riki can blink strike a guy and immediately get the tether slow going. And then they're already dead. The bonus attack and move speed never hurt, either.
The point of Riki honestly isn't that he's invisible, when people draft him in higher level games. Sure, it's nice to force your opponents to get detection early on, but most teams are going to get a gem at some point for vision control whether there's a riki or not. The best part is the Smoke Cloud. That thing is ridiculous. It's a massive aoe, lasts longer than Disruptor's ultimate, is pretty darn spammable, and also forces your opponents to get a BKB (or MKB) against a hero that does no magic damage.