You may be able to tell that my principle (only) complaint at this time with the 301 is the short maximum range for the HRM function to work. It is difficult for me to see any benefit to this.
It also has the ability to show you a track of your course after the fact. I just reset the thing after each workout - presumably erasing the data. For example, a friend showed me that it has the ability to "remember" past workouts. This has worked out well for me (except for the HRM issues described above). I've been attaching the 301 to my kayak with a (no longer available) Lincke Watch & GPS Holder - a piece of foam that the wristband wraps around - with suction cups that attach to your hull. I don't think I'll like that placement as much - since I'll have to look closer into the cockpit to check my speed/HR/distance. I guess that I need to try to somehow attach the 301 to my Lincke Foot Strap, which would bring it to almost exactly 35" from the chest transmitter.
Now that I am checking into it, while the chest transmitter manual claims to have a range of 9.8 feet, the 301 manual claims a maximum range of 35-43" (the 305 manual claims a range of 3m, about 117"), which completely explains my poor performance. I **THINK** that this may have to do with the fact that the receiver is so far away from the transmitter (i.e., the transmitter is on my chest, the receiver is four feet away). Sometimes the HRM display goes blank (i.e., consistent with having no signal) or reads low. My primary complaint with this unit is the HRM function. This estimate has proved reasonably accurate. When you turn it on, it gives you an estimate of how much longer it expects to go on the current charge. It claims to work for about 13 hours on one charge and that sounds realistic (the 305 claims 10 hours/charge). While kayaking, I always display the current speed up top, the mileage in the lower left and the current heart rate in the lower right. There are many potentially useful displays available. I haven't had to change the battery yet - after over a year of use. Also, unlike a Polar HRM transmitter, the Garmin transmitter has a user-replaceable battery. This is supposed to make it less likely that somebody's transmitter who is standing (or running, kayaking, etc.) next to you will interfere with your signal, giving you faulty readings).
However, Polar HRM transmitters don't work with it - the Garmin transmitter is somehow encoded. This unit comes with a Polar-like Heart-Rate Chest Transmitter. Since the unit is located quite a distance away (mounted on the hull of my surfski, just past my feet), it is useful for the display to be large so that you can see it! This larger display does NOT come at the expense of more weight - the 301 and 305 have virtually identical weights (78g and 77g, respectively). I seem to recall at the time (Nov 2008), there was a fairly dramatic difference. The 301 is (or at least was) less expensive (I got mine for $147, though the Garmin website currently is listing $199.99 for both the 301 and the 305). I got the 301 instead of the newer 305 because: This is an older model, replaced by the newer 305 (though the 301 is still available and apparently still being manufactured).