This is a timely topic for me. I have a Powertap and a Sports Instruments SI90HR, which don't get used concurrently. The Powertap is on a bit of a vacation right now, and the SI90HR is on my bike.
The SI90HR, which I think is discontinued, is great. You can look at speed plus heart rate, speed plus ride time or speed plus mileage. It also has a clock, average speed and odometer. You can set a heart rate zone and a little arrow tells you whether you are in, above, or below that zone. It seems to work doggedly well, although the curse of the GamJams review may be too much even for it. It's wireless, which is good. It cost around $90 a couple of summers ago. As far as it goes, it's perfect.
I'm heading toward an ever more minimalist setup right now, the explanation for which will explain my Powertap vacation. Objective information is great, especially in the winter when you are bored out of your tree on the trainer or doing testing to see where you are. During race season, I find it less good, sometimes even detrimental. I know the times for all of my benchmark sections like Ridge Road, Mass Ave up from Goldsboro, Angler's, and Great Falls, so I can know how I'm going on them. At this point of the year, I've ridden enough 20 minute intervals to know how hard I need to go. What I need not to have happen is to be a slave to the thing. Racing demands that you go with what's happening NOW, not what the number on the dial says. I think I've thought myself out of good spots in races because what was happening didn't align with what the numbers said.
I've often thought that training is like doing math problem sets, and racing is equivalent to a word problem or practical application of a problem set. It's all about transformation of what you can do "in the lab" to what you can do on the road. Too literal a line between the two, at least for me, gets to be too much.
There are two reasons why I bought a Powertap in the first place: my freakishly low heart rate, and my not knowing what "hard" means.
My max heart rate, to the best of my knowledge, is 178. Resting, it's in the low 40s. I consistently hear of people who never see less than 190 for a whole crit or other shenanigans like that. I was so profoundly weirded out by my comparisons to those that I couldn't convince myself that there wasn't some door I needed to unlock to get my heart to go that fast. It is so different to what other people talk about that I quickly realized that heart rate as a comparative measure was useless and therefore I couldn't answer the question of what hard is.
To learn what hard is, I needed some objective stuff. The Coggan measures for critical power was a Godsend to me. If you want to be about as strong as the average "x" racer, you have to be able to go "y" watts for "z" period of time. Seriously, when I bought the Powertap I was so far away from putting a finger on this that it was insane. Having an idea of that has increased my exercise literacy exponentially. It's very easy to correlate going "about 5 minutes max hard" with someone. A lot of people with deeper endurance backgrounds than I have will laugh at this, but sailing, windsurfing, tennis, squash and skiing don't really force you to think about this stuff. And Olympic windsurfers tested prior to the 96 Games, as a group, were in the top 5 of any sport (among Americans) when measured for VO2 max in Colorado Springs so kiss it if you think that shit ain't hard. Olympic windsurfing is freaking nuts intensively physical. Go do a wall sit (aka Roman chair) for two hours straight and you will know what Olympic caliber Laser sailors can do. Two hours.
So anyway, the Powertap helped me out a lot and it will I'm sure continue to do so in the off season, but to the many out there for whom it would be a benefit I will also offer the thought that sometimes it seems better just to put the black tape over that sucker and go. I'm about 3 weeks into this experiment and other than wishing I knew how hard I was going for about 15 minutes at Poolesville, I'm happy just to roll. The HR strap has stayed in the drawer lately too.