Quite useful indeed. Worth having in your team, if you have a spot that needs to be filled.
HP: 55 (Average)
Attack: 90 (Good)
Defense: 80 (Decent)
Special Attack: 50 (Poor, but unnecessary)
Special Defense: 105 (Quite good)
Speed: 96 (Good)
Notable moves: Shadow Sneak, Swords Dance, Hone Claws, Leech Life, X-Scissors, Shadow Claw
Weaknesses: Ghost and Steel. Only. That's pretty impressive, to be honest.
Immune to 3 types (Fighting, Normal and Dragon), making it quite handy to use.
Here's where it shines: As a set-up sweeper. Unfortunately, in game, most opponents go for the offensive right off the bat. In other words, you're guaranteed to be hit by a damaging move since turn 1. Thus, it's not as effective as it can be. Whatever. Mimikyu has solid defenses to set up at least once, and access to STAB priority (Along with STAB High Crit-Hit Ratio move), and the ever common Play Rough. To mix things up a bit, even Wood Hammer in exchange for a Heart Scale is a decent coverage move.
That's about it, sadly. It has very few moves it can take advantage of, with that dismal 55 Special Attack. For in-game, it can more than make up for that by being surprisingly effective. I'm not including the Toxic + Protect Stall combo as it's pretty useless in most trainer battles.
Final thoughts: Mimikyu is pretty decent, but not the best. There are some other choices you might want to consider before sticking to this one, but you won't be disappointed by it.