This is my second time buying this watch (I lost the last one in Glacier), and it's not cheap, so clearly I think it's worth the steep price tag! Honestly, there are too many features to cover them all a review.
I upgraded to this watch from the Forerunner 25, when all I cared about was having GPS to track my bike rides and runs, but the battery on the 25 didn't last long enough for long course triathlons. What's awesome about this watch is that despite all the extra features of this watch, it's battery lasts a long time. Depending on how much I'm using the GPS feature, I can get it to last anywhere from 4-7 training days without recharging - and I wear it all the time, even when I'm not training. I haven't tried it out for a full IM, but based on how it lasts for training, I think it would probably last.
In addition to options for tracking indoor and outdoor running and biking, pool and open water swims, there is a triathlon feature that allows you to transition between each sport in a triathlon. It allows you to see all your times - including transition, so you don't have to wait for race results. For pool swims, it will automatically count your laps based on what you tell it your pool size is. And for any of the sports you can program workouts in and the watch will tell you when you go/rest/etc.
The wrist heart rate monitor is one of the features I ended up really liking. It is definitely NOT as accurate as a heart rate monitor chest strap, but I can't stand wearing a strap on my chest while I workout and I've found that even if the wrist monitor isn't completely accurate, it is consistent, so you can still use it to identify what zone you are in. If you train primarily off heart rate, you should still use a chest strap, but if you have a Garmin heart rate monitor, you can sync them up on the Garmin Connect site (Backcountry sells a bundle with the watch and waterproof chest strap).
Another great feature is the ability to enable bluetooth on your watch to sync to your phone and automatically update the Garmin Connect App with your training information. This feature also lets you receive notifications from your phone on your watch. I thought I would never use it until I was waiting for an important call and really wanted to get my swim in. It's super easy to turn on and off depending on whether you want to be bothered during your workout or not!
Overall, it's a great watch for triathlon training and racing!