I looked at buying a HAC4 before finally getting a S720i. I had a S710 before that, but it was totally unacceptable due to the fact that it shuts down after 5 min with no HR. Useless on my bike rides. The S710i/S720i fixed that major problem.
There are a lot of problems with the S720i still - let me just tell you a few that I found :
1. the LCD has a very bad rainbow artefact. This comes and goes with various sunlight regimes, but on a bad day you can barely read the screen at all. Honestly. I have had two S720i now, both with this bad problem, and other users have also mentioned it. Polar says it is a manufacturing problem, and there is nothing much they can do about it. Frankly, if there was anything else around, I would take it back for this fault alone.
2. It suffers very badly from interference problems. And I mean - real bad. Taxis are a problem, as are tain lines (electric ones). Power lines also seem bad. Ok way out in the country, or in quite neighbourhoods.
What this means is that you can be sitting at the traffic lights, and you could be doing 140kph! Or your heartrate is 220. I see that a LOT.
I eventually went to the huge expense of getting the Power Output module simply so I could hardwire in the speed and cadence sensors. That fixed that - except the HR is still a problem of course.
The problem is it is very hard to filter out the bad readings - and the bad readings can totally stuff up your download results.
My Polar M52 hitting right next to the S720i has no such problems - works perfectly with no interference whatsoever.
3. The cycle computer functions are fairly poor, as other users have said. For example there is no Distance function as every other cycle computer on the market has. Totally amazing and rather a weird oversight. You do have Trip Distance, but as that does not reset itself, it is virtually useless - you always forget to reset it before every ride.
Update speed is a problem (very slow). Also all the cycle information is only on the top line - so you can only see one thing at a time, plus it is very small and hard to read. I find it almost useless for anything at all on a ride itself - except the heart rate.
While I found the S720i pretty useless on the bike itself - it has a VERY good upload to PC function. And the PPP4 software is VERY good. This alone is the saving grace of the Polar S720i - I can put up with all the other major faults as long as the PC software is so good.
Conversely, the HAC4 software is very poor. I am a software designer myself, and I was amazed at the very poor quality of the HAC4 software. It has not been updated for something like 5 years, whereas the Polar stuff is updated every few months. Totally chalk and cheese here.
This makes the HAC4 a very hard thing to justify buying. I also read many posts where users found that the mount was poor - and the unit fell off the bike fairly often. The HAC4 manual is almost unreadable - this says something, I think. We are all used to laughable transalations from Taiwan - but from Germany? It indicates to me that the designers have not spent very much time or effort on the whole package.
The only real saving grace of the HAC4 is that the US Postal team are using it this year for the TDF. And also the gradient functions (Polar has none).
The problem is - there is not much choice. The HAC4 might be ok, but it seems second best to the Polar. The Cateye looks very poor in comparison - barely worth consideration. The SRM looks pretty good, but it is way too expensive. As far as I know, that is about it for choice.
