I've been to damn-near every sushi joint in Mississauga, and I think Sehmi is my favourite. I actually had no clue it was called Sehmi sushi until a minute ago, always just called it 'that place on Winston Churchill' when begging my girlfriend to go.
The fish here is awesome, it's CHEAP, and the service is grea (seemingly flying in the face of all natural rules of eateries). From time to time the chef even sends over new things for us to try on the house. Sehmi has a small, quiet, homey feel to it- good for avoiding humans in flock. I think the fact that it has the corner lot in a little plaza has deterred others from going; I guess that's what makes it a gem.