What do I miss?
Quite a lot, although some features may sting more than others based on your needs and daily routine. Notably, the Android Wear watch doesn't integrate with the iPhone's communication functionality - so if you read a text message on your wrist, you can't speak out a quick reply, nor can you dictate a new message with your voice. Google Now just doesn't know what to do when you ask, unlike Siri.
If you get a call, you can accept or reject it on your wrist... but if you accept the call, it'll just open on your phone anyway. Many of the older Android Wear watches, including my LG G Watch, don't have a speaker for two-way communication - just the microphone for spoken searches and commands. Whatever the case, the watch opts to hand off that task to the iPhone once you've made your decision.
Crucially, Android Wear watches also don't have third-party apps on iPhone. As of now, you only have access to the functions provided by Google or the watchmaker - there aren't app experiences or games that you can play when paired with an iPhone.
Granted, watch-based gaming isn't everyone's cup of tea, but what about the helpful at-a-glance apps that we love on Apple Watch? There's none of that on Android Wear via iPhone, and currently no way for developers to extend their iPhone apps onto the Android watches. Hopefully that's something we see down the line, but I'd guess it's doubtful that Google and app makers alike will find it worth much hassle to satisfy what's surely a small niche of users.
Between what's on your watch and what the Android Wear app offers, you'll find at least 35 different watch faces to choose from - significantly more than the Apple Watch, at present. But you can't customise them. There's no photo face, whether with one image or a rotating selection, nor the ability to tweak the look, style or complications shown.
As for fitness, your functionality is determined by the watch. My two-year-old LG G Watch tracks my steps via Google Fit, but it doesn't have a heart rate monitor, so that's not available. Whatever your watch's functionality, the Fit data doesn't play nicely with Apple's Health as of this writing, so your data won't funnel into the native iPhone tracking app.
Is it worth it?
If you already have an Android Wear watch for some reason, the addition of iPhone compatibility is a great perk. Did you switch from an Android phone to an iPhone, but still had a watch? Receive one as a gift? Win one in a contest? Whatever the case, if you have a Wear watch, it's usable with your iPhone.
Sign up for CIO Asia eNewsletters.