I use Windows Phone and have a GV number (got the GV number before switching to WP; I would never have bothered otherwise).
It depends on what you're doing. Call forwarding? Doesn't matter. Someone calls your GV number, it will forward to any phone number, be it a smartphone, dumbphone, landline, Windows phone, anything.
The problem is third party apps. I use a WP app called Metrotalk, a GV app. It has an option that allows you to place calls using your GV number. It still uses cell minutes, but it works just like Google Voice on an Android phone. Is that feature going away? I think it will be but I'm not sure.
The other problem is texting. It seems as though third party GV apps (like Metrotalk) will no longer support outgoing and incoming text messages. There are some kludges you can use to get around this - GV does have a mobile site that is usable in the WP browser, and you can set up GV to forward your incoming GV text messages via email or text. Of course, if you have it forward the message via text, that kind of defeats the purpose of using GV to skirt around having to pay for a texting plan. But most people do have unlimited or near-unlimited texting so it's not a huge deal.
This also bugs me because my wife uses a third-party GV app on her iPad called Talkatone that allows her to make free phone calls over wifi using her GV number. That'll stop working this month as well. I'm not sure if the official iPad GV app lets her do that.