Power Tap?

Did you read the FAQ?
Each of them has their own pros and cons, none of us can answer which of them is most important to you. A couple of folks have invested a lot in one type of powermeter so they are going to chime in here in favor of *their* brand. I've used Polar, SRM and PowerTap and my understanding of the Ergomo makes it rank below those for the same reason you stated. The Polar is almost in the same boat - you have to either put a checked PowerTap or caiibrated SRM on the same bike to know if it's working (or just go on faith and and a stopwatch), and if you go that far, why did you bother with the Polar/Ergomo? You should also consider the cost/time for maintenance/upkeep which is not covered so well in the FAQ last time I looked(it's really easy to move the sensors on the Polar and SRM so that it's no longer accurate or not working and require adjustment unless you are very thorough on the install.

There are some other cons on the SRM - it can be very hard to mount the CPU on aerobars as the CPU mount only wants to go on one place, on the right hand side of the handlebars next to the stem or perhaps on the seatpost or sideways on some aerobars, and you make a really long term choice in crank BCD.

If you have multiple bikes you need to consider compatibility and ease of transfer.

What I've heard is the SL uses batteries that do not work under about 38 degrees, the Pro hub works to below 30 so far in my testing.

In 2007 all the PowerTap non wireless hubs are going to a different design so dunno what happens.

If you race at 170 pounds I think the wheel weight is a lower priority than your body weight unless you are really tall...

Natchez, here's the Training with Power FAQ that Woofer alluded to:

FAQ: Which Model is Best?

Woofer said:

What I've heard is the SL uses batteries that do not work under about 38 degrees, the Pro hub works to below 30 so far in my testing.

In 2007 all the PowerTap non wireless hubs are going to a different design so dunno what happens.

If you race at 170 pounds I think the wheel weight is a lower priority than your body weight unless you are really tall

At 6'2" and 170 lbs at race weight (between 170-175 usually), I'm running the Mavic OP 32 spoke with a PT SL. The difference between the 32 spoke PT SL and a lighter rim and the PT SL is at most going to be a couple hundred grams. Even if it were to somehow be 800 grams, we're still talking less than 2 lbs, which I can lose faster and cheaper.

Even so, my roads are pretty rough and I have had extremely reliable OP rims which is why I went back there.

As far as reliability goes, I've run my PT SL in sub 38F weather and in some pretty heavy rains with no problems at all. I purchased my PT SL in October. Some have had problems with theirs in wet weather or cold weather, but I've not had any at all.

I can also say my experience with Saris Customer Support has been second to very few companies I've dealt with in any sport/hobby/equipment purchases.

I'll finally disclaim this to say that I have absolutely no experience with any of the powermeter devices, but can't see that I'll every switch to another. I'll probably end up buying a second powertap later this year and building it into a lighter wheel if some finances fall into place.