power tap

what Aaron said although I disagree with the IM racing part IMO a power meter plus RPE are the best pacing tools, the longer the event the better. HR can be affected by many other variables besides intesity/effort while watts are watts are watts. you are either on target or not, no guessing or no what IFs cuz you have the data infront of you staring at you. watts and kilojouls can be great ways to make pacing and nutrition adjustements based on the race/terrain itself and the weather. I think PM+RPM are the best pacing tool for the bike and pace+RPE for running. Although a HR can be very useful in the absence of a power meter. (of course that's only my opinion and there are many athletes racing succesfully with either or tool)

Anyway, I have two power taps and I like it very much, however if you are planning to get a PT you will have to invest sometime in learning how to properly use it (or have someone who can help, coach, friend, etc) otherwise it will become a very expensive bike computer.

If you have questions, let me know. good luck

I use my PT for both training and racing my IM.

I have Zipp 404s for racing with power tap hub. When I race I triangulate my power, RPE and HR to determine my pacing. I do enjoy looking at my data afterwards to see how I paced.

I am researching into getting a PT or not.

Take a look at this study, it can be helpful for you

http://www.biketechreview.com/archive/pm_review_2.htm

you can scroll down several pages.