Though I haven't tested either of my Pebbles with iOS, it seems that Android is superior platform for Pebblers today.
From what I have gathered, notification control is very coarse on iOS, with somewhat of an all-or-nothing approach. However, on Android, the Pebble app allows you to manually select which apps can send notifications to your Pebble, meaning the Pebble can better fulfill its purpose of providing you only the most important notifications. If you're an app junkie like I am, you'll truly appreciate not having your watch almost constantly buzzing.
Next up, let's talk about the Pebble apps on each platform. On iOS, Apple seems to be very against Pebble, perhaps most evident by the Pebble Time's long wait in app review limbo to go live in the App Store, but also through what iOS allows Pebble to do with notifications and the OS in general. On Android, Pebble watches are given direct access to the system resources they need without hassle (primarily through granting the Pebble app access to notifications in Android security settings), making Android the more versatile platform for Pebbles. Despite the fact that the creators of both iOS and Android are in direct competition with Pebble through their wearable platforms (the Apple Watch and Android Wear devices, respectively), Google's openness with Android provides a better experience using a Pebble over iOS.
Finally, the big one: the Android Wear API. Pebble has integrated support for the Android Wear API into Pebble OS, meaning that Pebbles that have the Android Wear app installed on their phone can use their Pebble watch like an Android Wear device, and therefore extend the functionality of the Pebble beyond being a notification powerhouse. Naturally, given that the Android Wear API is intended for Android, and that Apple has no similar API in place, a Pebble on Android is much more powerful than a Pebble on iOS.
So to answer the question, the Pebble will work okay on iOS, however Android provides a much fuller and richer experience, and in my subjective opinion, makes Pebble watches worth their weight in gold, an ironic result given that even the latest Pebble watches carry the "Made for iOS" stamp on their packaging.