Small size ( it's really small!), single file. All functions, needed to me.
It's great, that i should not setup any soft on server, configure web server (apache. nginx, etc ). If you don't have any right for doing this things - adminer helps.
Simple interface - it's great.
Some times it's not easy to understand how make some actions.
Some times, i think, thats it would be great to see more colourfull or WEB2.0 interfaces.
In one project i haven't got any access to db : console/open ports or phpmyadmin. And i just copy adminer on server and run it. It fix small bugs, which makes a lot of bad thinks and company lose moneys.