I have a WP7 phone. Works for me. Sometimes the route calculation is a little slower than on an iPhone (that might be my service with T-mo), but otherwise, it's pretty solid. It also has a hands-free option, where, once you select your route, you can hit Start, and it'll read the directions aloud to you and re-orient your map each time you get to a turn. Bing Scout can be useful, too -- when you're looking at a map, if you hit it, you can look at local restaurants, bars, cultural stuff, etc. I'm not sure how complete their listings are, but it can come in handy.
I've had a WP7 phone for almost a year now, and I am surprised at how much I love it. To be fair, I had an early-model Android before, and have only occasionally used friends' iPhones. I LOVE the integration with the calendar and my myriad accounts (including work email), and really, really love that it shows me my next appointment and location on my home screen without having to log in. I also use Zune, and that's been pretty seamless too. The Office integration is pretty sweet, too, though I really only use the OneNote feature.
If there's anything I wish they would splurge on, it's a really great camera. My camera sucks (both hardware and software), and I am not holding out hope that the next phone I get will be any better.
Important disclaimer: I do work for MSFT, but my feedback is based on my experience with WP7 and other phones. I have colleagues who are Mac geeks, and swear by their iPhones, so each to their own.