I just purchased a mint condition Lumia 930 for $260 on Swappa three weeks ago, so $412 seems incredibly steep to me. There are also brand new models for sale on eBay for much less than $412. It's unlocked and an international model. I'll get HSPA+ here in the states which is fine for me, excellent if you're European/international; but to answer your questions:
Windows Phone OS is really fun, fast, unique and just flat out great. I really am enjoying it a lot (currently have a HTC One M9 and an iPhone 6 Plus as well). It's clean but not absurdly dumbed down like iOS. I think more veteran users or fans of Windows OS will tell you the market place isn't "that bad," but... Realistically speaking, yes it is that bad and it will continue to be bad in comparison to Android and iOS because of developers ignoring it very heavily. It's not the worst thing in the world and for the most part you will find all the main, popular social media and music apps on all platforms. But those niche apps that just change your app-life will more than likely not be on Windows Phone or kept in un-updated, shabby editions. Not to mention the ONLY mobile game I have ever played (Fallout Shelter) is currently only available on iOS and will shortly be on Android. Windows Phone will probably never get it and that hurts.
I know a lot of it isn't the OS or MS's fault, but it's a problem nonethless. I hope the market place improves, but once again (with all due respect) nobody on this sub reddit can really know for sure, especially the level of improvement.
The camera is fantastic. The battery isn't bad. Like I said, 5.5" phones or bigger I'm used to Note 4, One Plus One, Nexus 6, iPhone 6 Plus battery life they all have excellent batteries with the OPO and Note 4 boasting ridiculously durable battery life. I do need to watch my brightness settings very frequently which is something I'm not used to doing ever, but this little guy does a good job staying alive. I can't complain. I have a HTC One M9 and that thing just drains.
And honestly, I'd say the worth depends on how much you pay for your phone. If you can get a good deal, of course! For example, if I buy an iPhone 6 Plus at retail value for $750 and the iPhone 6 Plus S (let's assume they keep the 'S' moniker) launches in 3-6 months and my phone immediately drops in value - I may have made a mistake.
The good thing is Lumias don't drop in value much, at least the higher end 1520, 1020, 930s. And of course Windows Phones are worth it, not to mention the 930 will get Windows 10. This is my first Windows Phone and I'm really loving it. I only use iOS to play Fallout Shelter on an iPad and Snapchat on my iPhone.