I have been using the 710 for about a year now, and am very glad I purchased it vs. the 510. A number of my (road)teammates have 510's, and the faults are obvious and very telling...
-the 510 is basically good for only one downloadable exercise at a time. If you finish your run, turn off the hrm, drive to the pool, turn on the hrm, you just nuked your 1st (run) file... you can download only a "summary." This alone makes the 510 useless in my book.
- IR vs. SonicLink - after seeing a laptop fly across the room at a team training camp, after 15 minutes of muffled sonic-link inspired cursing, I'm sold on IR; I have had zero problems with it. Perfect downloads, every time. That's worth 40 bucks to me.
- Software on the 710 is not only better than the 510's, it is pretty darn good on it's own merit; this is now my main method of tracking my training. It's very easy to use, and while it does track a lot of info, some of the graphic representations available do a very good job of distilling that info into a novice-usable format. Altitude v Cadence v spd v HR on a graph is pretty darn useful, and far from rocket science.
- I think that people who experience dropouts of HR function while using this HRM need to experiment with electrode cream. The 710 is the ONLY HRM I have ever used that I have not had dropouts with, even under high tension lines.
- The wrist unit IS large, and I thought it would be a problem, but it hasn't been - I even wore it for all my Cyclocross races this season, and if it didn't get in the way there, it won't.
- The ability to add power, should Polar ever work the bugs out of the system, is a plus, but I haven't gone that route (see the rants on the wattage list for more info than you could possibly stomach on this topic.)
that's probably about 2 cents worth...
Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
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