I use Bluetooth on Windows all the time (like right now I am listening to music on Bluetooth headphones.)
The performance of individual devices for Bluetooth can vary widely.
A year ago I built a "gaming PC" which has a relatively big antenna that looks like the "ear" of a giant robot from a 1990 anime and I can walk all over my 2200 square foot house with my V-Moda headphones and never hear a dropout. I have a Dell Latitude laptop which was strong enough to connect to headphones through 10 (flimsy) interior walls at the office; it used to run Windows but it runs Linux now and still does a great job connecting to BT mice and headphones. On the other hand I also had an Alienware (also Dell) laptop which had dropouts if I turned my head sitting in front of the machine.
Other people think I'm crazy but I've never had iDevices of any kind be able to play music on BT speakers while streaming music over WiFi (even friends visiting can't make it work.) My current "on the go" device is an iPad which my major complaint is that I have 3 BT devices I want to use with: keyboard, mouse and headphones, but I can only get 2 of them to work reliably at the same time.
I think it comes down to this: a PC or phone could have very different electromagnetic performance based on the design and some are much better than others. You don't see it advertised or even tested in reviews so you can't just shop for a computer which has good BT performance. Similarly different devices are going to have different performance levels and it all can vary based on details... My V-Moda headphones really work great with my Yoshinon (e.g. the physical home of YOShInOn) but with some lesser headphones you might come to the conclusion that Yoshinon sucks.
Because the standard is so complex, all kinds of implementation details, even in software, could have a big impact. For instance, if you are using an audio codec with a lower bit rate you are making less demand on the radio and might have better reliability, but sound quality would be better with more bits. I love quality sound but if I had the option I might trade quality for reliability in range but... I don't get this option.
As far as frequency, Bluetooth uses the same 2.4GHz band that WiFi started out using. You'd better believe that Bluetooth is degraded if a lot of other devices (even other Bluetooth devices) are using that band. Sometimes I wonder if the whole "convertable" laptop thing was (a) an attempt to confuse flight attendants (do you have to put it away?) and (b) the shadow of the consumer electronics show where there are so many half-baked gadgets at such a density that you'd be insane to try to demo anything that uses BT... With the effect that devices aren't really designed with the BT lifestyle in mind because even if it worked at your home and office they couldn't demo it. (900 MHz has similar problems... The band is practically dead in 99.9% of America but go to Palo Alto and you find every Stanford grad has a bunch of half-baked gadgets that make my scanner scream and would convince any startup that the band is so crowded you can't use it.)
I'd also say the industry just isn't thinking about it in the right way. I wish there was an "infrastructure mode" for BT where my headphones could roam throughout the house and the data goes over the Ethernet and comes out through one of my WiFi hubs. It's the kind of thing I could probably get sorta-kinda working in a month or two with one set of headphones but to make it completely reliable with all the different devices out there and under all circumstances looks hard enough I can see why it hasn't happened.