I'm using a PowerTap SL+ coupled with a Garmin Edge 800. I love the Garmin, I can display all kinds of data and I can download maps for turn by turn directions. It's also great for downloading where I rode, power, HR, speed, cadence, elevation, grade, temperature (usually off by 5-10 degrees), calories (need to calibrate mine, seems off), etc...
Last night I rode from my house over to the business park where we ride in circles for about an hour every Tuesday night. Then I nursed myself home. Looking at the data it really doesn't do justice to how tired I was. I think I got stopped at about 6-7 lights on my way over and didn't get stopped once on my way home, so the average speed was much slower on the return trip. When I'm stopped the timer keeps running and my average speed slows, I can change that but I like it that way. It's about 11 miles from home over to the business park and 11 miles home. Last night's training race was actually slower than usual, probably due to the heat and the group was a bit smaller. I've seen averages in the 28's and 29's, but usually it's about 27.5. I think last night was 26.7 or so.
Here's a link to the Garmin Connect data from last night, there is a map of where I rode. Lap 1 is home to business park, lap 2 is warmup lap around course, lap 3 is training crit, and lap 4 is ride home.
http://connect.garmin.com/activity/97138302