[Windows Phone 8 Q&A, Help & Troubleshooting] GoodDayToDie:
"Is it worth it" is a personal question - it depends on what you value, how much you can afford to spend, what phone you're thinking of (there are at least six Lumias in active production and at least one older model still being sold), what deal you can get it for... however, you're asking a forum full of Windows Phone users, so the general answer would be "yes".

Is it better than Android or iOS somehow? Yeah, a few ways. The interface is much more dynamic, presenting at-a-glance info and requiring less popping into and out of apps, plus the lock screen is pretty configurable. The battery life requires much less shepherding on WP8 than on Android, and there's also none of the malware found on Android. The built-in Office apps (Word, Excel, PowerPoint, OneNote) are better than any that I have seen for either Android or iOS. Xbox Music Pass (formerly Zune Pass) is very sweet, although it's not available in all regions; check this. Similarly, Here Drive (formerly Nokia Drive) is better than other navigation apps (built-in or third-party) that I've tried, but may not be available, or at least not free, in all regions. Developing for WP8 is really easy and you can use a large number of languages. The software keyboard is excellent; I prefer it over the iOS one (massively) and over most Android ones. The integration of Skype calls is really nice. The ability to switch between Live messenger, Facebook chat, and SMS/MMS in the Messaging hub is very cool. The fact that you don't need to install iTunes is a plus on Windows (unless you already use it... eww). All Windows Phones are required to have a double-action camera button (half-press to focus, full press to shoot) although you can also use tap-to-shoot. There are lots of Xbox Live-enabled games, many of them quite good, which are not only fun but also earn you GamerScore on Xbox Live. The Xbox Live integration also lets you message and game with your Xbox Live buddies, although you can't generally play between phone and console directly. Xbox Smartglass lets you use the phone to control, view, and interact with the Xbox 360. Generally speaking, phones are updated faster than on Android. If you let us know what's important to you, we could possibly list other advantages...

The data plan is a carrier thing, not an OS thing, but will probably be the same price and features as other smartphones. That's something you will have to research yourself, though.

I like the Windows Phone devices I have (an HD7 running WP7, and an Ativ S running WP8). They do have some limitations - WP8 has not yet been hacked much at all, so there's a lot less homebrew for it than there was for WP7 - but overall they are nice. They get good battery life (the HD7 not anymore, but it's 2.5 years old), have good (if not all-encompassing) app selection, and have a number of nice features (WiFi tethering built in, for example). The main thing keeping me on Windows Phone is that I have a legacy Zune Pass (the kind that allows 10 DRM-free downloads a month) and therefore I really want a phone that will let me stream or download all the other songs so I can discover new stuff and decide what I want to use my credits on each month (before I use them).