Here's my overview of the OS:
Many improvements to Windows Phone 8 that make it more competitive to it's competitors
Many visual improvements make it one of the better looking mobile platforms. The live tiles are far more customizable now.
It is noticeably faster than it's predecessor. Apps open up quickly, and multi-tasking is faster.
Frequent updates mean you'll see things improve in real-time. But there's a catch... (see cons)
Cortana is out of Beta. It's actually pretty decent now, it can manage your phone for you, it can look up anything for you, it can hold simple conversations with you... all that is great, but it's a little suspicious cause it requires you to share your location (via GPS) every single time you want to use it... I can't think of any good reasons for it being a requirement of it.
App library still falls short of Android & iOS (though that is supposed to change soon due to easier app porting, but it is still upto individual app developers to decide if they care enough about the platform to port their apps to it)
Automatic updates can't be turned off (Just like Windows 10 on PC)
Not bug-less in it's current state
Edge Browser is not stable (Crashes occasionally on heavy websites. Video streaming issues, etc)
Here Maps is still useless in Pakistan. The only way you can currently use Google Maps on your device is through your web browser, which is not that great of a experience.
New Live Tiles, App Drawer, Notification Menu & General Settings:
This is how the new Live Tiles look like. You can customize their transparency, opacity, size & color in the personalize menu. The background "wallpaper" can be set to anything as well.
The accent colors are now available in a lot more variety as well. The themes available are still Dark & Light (White text on Black display or Black text on White Display).
The App drawer is slightly better arranged, and scrolling is smoother.
The new notification menu is awesome. It has everything you could want to put up there. It's honestly even better than the one on Android.
The settings menu is streamlined and much simpler and straight-forward. Everything is arranged as it should be. All settings are easily accessible.
New & Upgraded Apps:
Internet Explorer has been replaced by Edge Browser. Is it any better than IE? Yeah i think so. Is it comparable to Chrome or just about any well-rated browser on Android? Nope.
Cortana has been upgraded as well. It is more intelligent & can hold conversations better. Not perfectly understanding though for non-native speakers.
Microsoft has finally included a proper file manager on their OS. It is not the same one that was optionally available for Windows Phone 8.1, this is a whole new one with better functions.
Microsoft Office Trio has been upgraded as well. It seems quite decent for that unfortunate moment when you must rely on your phone to edit a document. You can create them too, but they'll be a lot more hassle to make on your phone.
Other smaller things worth mentioning:
There's a new one hand mode. Pressing and holding the Win key turns it on. What it does is that it divides the screen in two boxes. The top box remains blank. The bottom box shows whatever app you're viewing. It is certainly handy for larger phones which are not very single hand usage friendly.
The lock-screen has been slightly tweaked as well. The PIN unlock screen is transparent now. It kind of looks a little better.
The UI of all apps has been simplified similar to Google's new format.
The keyboard includes a joystick now (for correcting spelling mistakes).
It is a noticeable upgrade over it's predecessor. I predict that Windows Phone user share will see some visible increment. Would it be able to take over any of it's competitors? I don't think so, not for a while anyway. Is it better than it's competitors? Depends on your preferences. Is it worth trying out? Definitely.
I can say this with certainty that Windows 10 Mobile is a fully capable mobile OS, it doesn't feel inferior anymore.