The Focus is a capable phone, but yes, WP7 is limited right now -- you'll know it the moment you try to switch away from Slacker radio and the music stops. If you can live with single tasks (except for music, of course) and the other current features, you're set.
But it's always a gamble to buy a device based on what you hope will happen, because there's a very real chance it won't happen soon enough. The iPhone 4 does whole lot more than the Focus, right now. And unlike the way it was with the iPhone in 2007, you're not getting a fundamental improvement in interface to make up for the feature gap!