I own, or kind of own (it is on an extended loan right now), a Garmin 405. While it was a fine training tool, I found that it's value was limited. At least with regard to training for triathlon or for an Ironman in particular, that is.
First off, it wasn't waterproof and while some people did swim with theirs, the few times I accidentally jumped in the shower with it on, the water made the touch bezel freak out, which left me less than confident that a swim was safe territory. Secondly, when using it for long bike rides, I found the battery life would often come up short. If I had a long ride and a run on tap, it was a one or the other situation. Another issue was its size, don't get me wrong, I understand and appreciate Garmin's effort to make the unit small and as unobtrusive as possible. But the small screen, with 3 data fields, was pretty hard to read at times, particularly when going 20+ mph on a bike. Finally, given the water issues, battery life issues, screen size and the time it took to link to satellites, it was no good for racing triathlon.
Well the folks at Garmin have addressed those issues and done so in quite good fashion in my opinion, with this new unit.
I picked up a new Garmin 310XT at TriSports.com, and after time to give it a good test out in AZ, it has proved itself pretty awesome. It solves all the issues I had with the 405. Its H2Oproof, its got insane battery life, the screen is big enough to easily read up to 4 data fields and yet it still manages not to make you feel like your running with a microwave strapped to your wrist. The satellites link ultra quick allowing you to leave it in transition, turned off and then quickly turn it on and have it sync before you are out on the bike or run. The 310XT is also Ant+ compatible so if you have a powermeter that also is, you can have it display a power field on the watch. In the triathlon race mode it will switch from your custom race data fields for the bike to run in the push of one button. I haven't had the time to figure out all its functions, but I am thoroughly impressed thus far. So, if you're in the market, and if you don't have the 310XT already, I suggest you put yourself in the market, hit up TriSports.com and get yourself one. Use my coupon code IMIKE-S for 10% off. Every little bit helps.