I've used the BB10 OS, and to be candid, for people who use their device primarily for work, it is by far the best OS out there. I really mean, by far. Integrates seamlessly with Exchange (which many/most businesses use) right out of the box. The email and calendar apps in particular are far and away better than anything iOS and Android have. You can view pretty much any file right out of the box (try viewing a .tiff file via email on the N5 for instance), can view other people's calendars (whether they are busy or not busy before you book a meeting), the BB Hub is exceptionally functional and incredibly efficient for perusing emails and your social accounts (LinkedIn, Twitter, etc), the multi-tasking capabilities embarrass iOS and are still better than a Kit Kat (which is pretty decent). The gester/swipe functionality once you've gotten use to it is also very efficient and well thought out and the touchscreen keyboard is awesome (better than iOS which obviously isn't saying much). Removable battery is useful, and battery life is much better than most (mind you, the Z30 in particular weighs a ton because of it). If I recall it also permits microSD card storage. Very useful "notification settings" which are much easier to switch between than the Android devices.
BB has most of the apps you'd use for business and enough for news, but pales in comparison, obviously, to Android and iOS, which is where it falls down for most people who want to also use the phone more for personal use.
Unfortunately BB took too long to get the OS out, and when it did come out it was very buggy, just ticking off users even more, and the hardware was questionable (and still lags behind in terms of specs though you'd likely never notice this in real life use). They have bungled pretty much every release they'd sent out due to this, and have been late pretty much every single time as well with releases in general.
At this point people (including me, I'm ashamed to admit), are pretty much ditching BB because let's face it, people just look at you oddly these days if you don't have an iPhone first and foremost, and second at least an Android. They either assume your company won't permit it, or you need that keyboard because you're afraid of touchscreen devices.
But for users who want a device primarily for business use exchange email/calendar/phone/basic social accounts/basic web browsing, it is far and away the best option.
The N5 has a better screen and processor (again, not sure how evident this would be outside of when you are gaming on your phone and/or using an exceptional amount of apps at once); personally I love the form factor more than the Z30; much more ability to customize your phone (widgets, apps, etc); an incredible array of apps permitting an app for just about anything (well, except a decent Exchange app!).
Interestingly a work colleague who heads up IT went from an iPhone, of which he had been a user basically since the iPhone's came out, to the N5 for a short period of time before switching back to the iPhone and I told him he should try a BB10 to check out how useful it was for business, which of course he scoffed at repeatedly. When the Z30 came out he decided to give it a go, hated it for 2 weeks, and now has gotten rid of his iPhone and N5, and is essentially touting the Z30 to everyone.
If anything, after having used the N5 for several months, the Z30 is the one phone I'm considering getting (was on the Q10 previously and tried an iPhone for a very short period of time) and leaving the Nexus 5 fold due to how much more efficient the BB10 OS was for work.
Anyhow, for non-work purposes (and maybe even I'd go as far as to say for non-Exchange Email work), the N5 is a kick*ss phone (save the battery issues). But for work, the BBOS in my opinion having actually used both quite heavily, wins hands down.
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