The story starts off with the main guy sitting on a toilet and this random girl showing up above the toilet stall.. in the men's bathroom. Priceless.
The comedy is fabulous without being total gag; it's well-done and fits the story perfectly. The development of the main character is clearly seen throughout the whole story; it is really cool to see him change over time. The main girl goes through some change too, but not quite as much. In fact, all the characters develop at least a little in the story.
I'm a big fan of Hiro Mashima, mangaka of Fairy Tail & Rave Master, and this mangaka worked with him, so it's a personal big plus. ^^
If you like school life, lots of comedy, good art, a little bit of delinquent action, and just a fun read to pass time, I highly recommend Yanki-kun to Megane-chan.
edit: what I wrote above is still true; however, that ending... was really disappointing. Part of it wrapped up nicely but the other half just kinda.. sunk. It doesn't make much sense why the mangaka went this direction. :/ Still a well-developed character with the main guy though.