Currently,the only supported devices are, the Lumia 630, Lumia 635, Lumia 636, Lumia 638, Lumia 730, & Lumia 830. Microsoft has promised that more handsets will be added to the future in the future.
It is said that Windows 10 will be supported on almost every device, so it is safe to assume that every device mentioned below will get it at some point, some with some features removed but still it will get it. But today, only the 6 devices mentioned above will be getting the update.
Talking about the First Developer Preview of Windows 10 for Phones, Microsoft also went on to call this a “very early” preview, which seems like they are expecting to see some bugs. This might make your device unstable, but enthusiasts can definitely try it out and see. Check the Blog post by Microsoft to learn more about the First Preview. To learn more about Windows 10 for Phones, checkout my previous article here. If you want to try Windows 10 for your PC you can Download it here.
Now if you are an enthusiast, you can install the preview of Windows 10 if you have a supported device as mentioned above. Checkout my Tutorial on How To Install the Windows 10 Technical Preview on to your Smartphone here.