They're good, but they are costly.
Not only do you have to buy the console for a few hundred quid, but none of your 360 games will play on it, so you will have to purchase Xbox One games.
The majority of Xbox One games on digital download will cost you £50-60, and similar for new physical copies. You can get them cheaper though, if you're smart. I got Destiny for only £20~ from an Amazon seller.
Furthermore, most packages don't contain a controller charger or a second controller, and these can be quite expensive.
You can subsidise some cost though if you get a non-Kinect version, which are cheaper. You can later buy a Kinect and it will work with the Xbox One.
The Xbox One charging cable is basically a USB-to-microUSB phone cable (I've not tested if a normal one will work, but I suppose it should), which also means you can just plug it into your computer and use it as a controller for that too, which you couldn't do with the official Xbox 360 charger kits, which didn't transmit any data.
The Xbox One is a lot quieter than the 360, in my opinion. I don't mean volume sound, I mean the mechanical sound of it's fan, so that's nice.
Navigating the Xbox is a bit unusual at first, but kind of self-explanatory. It's not dissimilar from the Metro style of Windows 8/Windows Phone.
The big downside though is just how limited you are when you're offline.
You can't view your pins (you can pin your favourite apps/games so you don't go looking for them), you can't view your friends list, your achievements, or pretty much anything besides your Games/App Library and whatever you're playing.
You can contact friends playing on the 360 and see what they are up to, but I'm not sure if you can enter an Xbox Live party with them.
360 friends can't see what you're doing on the Xbox One though (unless that has changed since I last used a 360), you'll appear online but they can't see what games you're playing nor your Xbox One achievements.
Messaging via Xbox One is really nice though. You find your friend, type your message and press enter, and it gets sent straight away, rather than asking you if you're sure, as it was on the 360.
The Xbox One also contains a BluRay player, so that's very nice.
On the downside, Xbox One doesn't have all of the apps that were available for Xbox 360. A lot of the big ones are available, like Netflix or Amazon Prime, but some aren't. (I think Sky isn't).
Xbox One games, even remakes of 360 games are generally very nice looking, provided you have a nice HD TV of course.
Xbox Live sharing is a very nice feature, especially if your family gets multiple Xbox Lives, as it means you all get Xbox Live for the price of one.
Your downloaded games can also all be used by each other, provided the owners-account is also on that Xbox One. (It can be on multiple Xboxes).
And as you've seen me complain about, the Games for Gold for Xbox One are generally a bit crappier than those of Xbox 360.
This is mainly because there isn't a massive library of Xbox One games yet, and most of the really big games are new and obivously worth a lot of money.
So instead a lot of brand new indie games go for free instead. I think a lot of them probably have it in their contract for being put on the Xbox Live Marketplace.
Sorry that's such a jumble of words. That's just my musings, in no particular order. Otherwise I'd have probably organised it into a nicer pros/cons list.
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