There's about a dozen iPhones using Exchange ActiveSync for e-mail in the small department where I work (we have a mix of iOS, Android, one Blackberry holdout, and a WebOS holdout who just ditched his Palm Pre for an iPhone 5, finally). I have iOS6 running on the iOS devices, a mix of iPads, iPhone 5 and 4/4S models. All have been working consistently fine for push e-mail, no need for resets. The iPads and 4/4S devices were upgraded OTA, and only one 5 was setup fresh; the rest were restores from their owners' previous 4/4S backups.
Beyond my department, there's about 15,000 employees in various other departments/locations for this same organization, with a mix of Android, iOS and Blackberries, all using Exchange. We have support channels to comapre notes if any IT staff notice issues, and so far there's been no mention of this.
Maybe it's something about how we configured and update our servers. Who knows?
That's a lot of setups. How would you say iPhones compare with Blackberries as Exchange clients? I seem to get notifications a bit later on the iPhone than I did on BB, and it seems to just drop off the server altogether occasionally (in areas where service isn't great).
In my case, as far as push speed goes, it's a wash. Sometimes our remaining Blackberry user gets e-mail notifications a fraction of a second quicker; other times the iPhones get it a fraction of a second quicker.
One thing is for sure: Blackberry devices require more middleware, additional server infrastructure, and added cost in order to get them to work directly through BES (as opposed to BIS which kinda negates the advantages of push).
From an objective IT standpoint (and coming from a former Blackberry user, long ago), it's added burden with increasingly little justification for that added burden. Where I work, we're certainly having a hard time justifying running a whole extra server for one Blackberry user, even if they are pretty high up the totem pole.
Also, the "better security" argument that RIM flaunts stopped holding water back when the 3GS came out, with its support for MS Exchange integration with encryption. Now, having to route all traffic through Waterloo seems more of a liability and a single point of failure, than a benefit.