My wife was a staunch BB user who finally got fed up with Blackberry's stagnation and decided to switch. She tried an iPhone (twice), but returned it both times and went back to her Curve. Too much of a mindshift there (from a business device to a toy). Recently she picked up a Lumia 920 at the same time I did, and she's been happy with it so far. If you're an email power user, you're going to be disappointed with anything else out there at the moment. Blackberry nailed mobile email - their functionality, feature list and push tech is astounding and incredibly reliable and nothing has been able to touch it. If you switch away, whether or not you get reliable push email is wholly dependent on which email service you use. We both use Gmail and Outlook.com right now, and both are great on WP8 - zero complaints with speed and reliability. iOS was horrific with Gmail until recently, and most POP3 accounts had to be jankily routed through Gmail and even then it only checked the server once every 5 minutes at best, and usually closer to ever 30 mins, so it was nowhere even approaching "push". This is something you never had to worry about on the BB because of BES. I haven't tried a POP3 or exchange account on WP8, but you might want to check with those who do if you use or rely on that. Be prepared to do without things you take for granted now like "mark all as read". Maybe these features will come, maybe not.. and maybe RIM won't include them in 10 - who knows?
You'll also miss BB's battery life. The Lumia is excellent, but even its impressive two days it doesn't touch the week (or sometimes two) of battery life on a charge my wife got out of her Bold. Granted the screen was tiny and the phone didn't have many battery-sucking features, but still - it's a change from not having to worry too much about battery to living from charger to charger (this is true for any modern smartphone at the moment).
The cool thing about going from BB to WP8 is that since damn near no apps were written for BB, it won't be a huge change to the current state of WP8 (which will evolve quickly, but right now it's pretty lean). It's much harder coming from iOS or Android where you're used to a huge variety. If you do use any apps, make sure whatever platform you go to has equivalent versions, or you can deal without for the time being. Coming from Android, I'm missing Spotify, Google's turn-by-turn directions and Google+ pretty badly on the phone, but I like it enough to stick with it and wait. It's only a matter of time before the major apps appear for it.
So really, it's a matter of what kind of user you are, what will annoy you or what you can live with, and what you expect to be able to do with a phone. It might help to list all this out and make your decision based on what hits all your criteria whenever you're ready to make your move.