Hey. I had a HAC4 for six months or so (recently went to power training and a Powertap Pro). There are a lot of mixed reviews. . .I thoroughly enjoyed mine, had no issues with it at all and found it reliable. There are a few 'quirks' with it though. . .so, here are some observations:

* Never had an issue with reliability, wireless, etc. It always recorded and was accurate
* The software is useful but a little cludgy. There are 3 separate programs that you can use depending on what you want to do. . .They all work but have quirky interfaces and were not written by the same people
* The mountings were robust and never had and issue with looseness, etc. (I understand some early units didn't stay on the bike so well. . .)
* The altimeter feature is useful and you can print an nice ride report with elevations. ..not sure it is absolutely accurate but it is relatively accurate
* The data is only collected once every 20sec. This can be a problem, depending on what you are wanting/doing
* Power is close to useless even relatively so. . .totally inaccurate
* It is a good value - you can get everything (computer, mounting, computer sw/kit, cadence kit) for under $300 on ebay (including shipping)

So, depending on what you want it can be a great tool