This is Rus, from your server. I registered here a little while ago intending to post first in the Introductions area (and maybe start learning mapping soon after that) but I was directed here first.
Anyway, here's what I think:
The gameplay is pretty wild! Tiny Spies, tank blitzkriegs (52 tanks, most of which are fast and fragile), giant Medic rushes and giant Spies, punctuated with the occasional regular wave of Soldiers, Scouts and what have you...the only complaint on my end is that the constant barrage of exploding tanks is horrible for my framerate, and the tanks like to swerve around an awful lot. Consider widening the tank path, or the whole map if you need to, and removing all unnecessary props and areas (including the secret room; I like secret rooms but they are expendable).
There's also a pretty big line of sight when exiting the spawn; I'd consider doing something about that; you might have to rework some areas.
The aesthetics also need work. Major work. As it is right now, it looks like you've been using placeholder textures, and in some cases props. Try checking the official MvM maps for ideas on how to touch it up.
I agree with takabuschik: you could make this a tongue-in-cheek Mann Co. Chair Disposal Site, conveniently located next to the Grand Canyon.
Overall, it's pretty good right now. Feel free to criticize my criticism, because I'm fairly new to mapmaking myself (in fact I still need to actually get to it!)