First of all, I have a lot of experience with Sigma's older bike computers, namely the previous generation BC 2006 (circa 2003). Those wired computers proved to be extremely reliable, surviving many years of use, with only a battery change every 3-5 years. But the old computers didn't have an altimeter, so I had to carry a separate altimeter, which was a bit of a bother, so when Sigma announced the MHR 2006, with both altimeter and heart-rate function, I resolved to get one if I could find one at a reasonable price. REI had a massively good deal (20% off) at the time, getting it down to a price that was under $100, so Roberto and I both bought one.
The computer has capacity to do two bikes worth of distance and time measurements, as well as showing a grand total. This is accomplished by keying the computer to a given sensor, which means that you'll have to buy a second sensor unit (about $30) to make use of this functionality. The older sigma models had a button on the back that you pushed to flip between two bikes, and it turns out that I actually like that better, not just because of the savings in costs, but also because if you swap wheels with different tire sizes on the same bike, you had the option of having a different setting for each wheel size, while on the MHR2006 you have no choice but to tweak the wheel size settings.
The nice thing is that the MHR2006 was designed for you to easily change settings without having to poke at setting buttons with sharp objects (which was what you had to do with the old models). The setup is extremely consistent, with one button to go into settings mode, one to scroll through the options, and the other two to tweak numbers up and down. Well thought out, and sensible. Even better, settings and statistics are retained between battery changes, so you can swap batteries without losing data. The only complaint I have is that when you switch between metric and imperial, the computer doesn't automatically recompute your stats in the new regime. Seems like if you're going to brag about a 32-bit processor you might as well include some firmware to do basic calculations. Maybe they don't have floating point numbers in that thing.
Installation was an easy task, with the magnet the only item that needed a tool (which is provided in the box). The magnet screws onto a spoke, the sensor goes around the fork blade with a rubber band, and the computer mount goes either on the handlebar or the stem, also with a rubber band. That's it. The box also includes a strap for hiking (you get no distance information but you do get altitude and heart rate), a heart rate monitor bra strap, and a pin tool to open up the battery compartment (woe to you if you lose this tool!).
I only used the heart rate monitor strap a couple of times. No matter how hard I try, I can't get used to wearing a bra, which is why I'm not a serious biker. The altimeter, however, proved extremely accurate during the tour of the alps. When calibrated, it read within a few meters of the summit sign, except when the summit sign was wrong (which happened often in Italy), where the maps would then usually agree with the altimeter but not the summit sign. One key of the accuracy of the altimeter is that the unit includes a thermometer, which is used to adjust the altimeter readings.
The thing that bugged me most about the MHR2006 was that after 4 months or so of use, the computer would occasionally get stuck, reading 0 or reading a fixed number for some time and not registering miles. Since the unit has a low battery warning, I assumed that the unit was being unreliable, and not that the battery was dead. In fact, the transmitter does not have a low battery warning! So all that time, the transmitter was intermittently sending out signals, while the head unit was heroically trying to interpolate between the signals. Believe it or not, while the battery was somewhat still delivering power, this heroic interpolation came pretty close to being quite accurate! Replacing the battery today made all the problems go away, and now the unit is behaving correctly again.
Having a transmitter battery only last effectively 6 months seemed pretty low to me, but glancing at the Amazon reviews shows that apparently the unit ships with pretty bad batteries. Sigma sells a stack of 10 CR 2032 batteries at a time, apparently indicating that this might be a power hungry unit. Certainly, it seems like my days of replacing a battery every 3-5 years might be over, if I decide to settle on this unit as my main computer. At least, all the components that need power (head unit, transmitter, and heart rate monitor bra) use the same battery, so you have no need to stock different sizes.
Another annoying thing is the trip meter. It has insufficient range for a multi-day tour. Basically, if you run over 999 miles or kilometers, it zeroes out and starts again. Not a big deal, but the first time it did that I didn't have the manual handy and thought that the unit had somehow reset itself (or that I had done it by accident!). Probably not a common complaint for most cyclists. There's also no download feature, so you can't download the data and graph your heart rate against incline or route, for instance, but units like that cost 3 to 4 times as much as the MHR2006, and have even worse battery life, so I think that's a reasonable feature to leave out.
All in all, I'll give this computer a cautious recommendation, with the above caveats. My biggest complaint is having to carry the pin tool and extra batteries on tour (especially if the weather might turn cold, since the battery's voltage can drop dramatically in those cases). Otherwise, the unit seems fairly accurate and reliable.