I recently got a Joule 2.0 after having issues with several Garmin 500 units I tested. Here's the quick (or slow) and dirty:
Reliability: JOULE: My garmin regularly froze (when hitting the lap button for instance), and dropped signal from my quarq power meter, from the speed sensor, and was totally flaky. The Joule has been rock solid under the same situations. The Edge showed a bunch of zero wattage values that I thought were me -- it turns out they were dropped signals, but I don't see this on the Joule.
Size/form/mount - Tie: garmin is a litle smaller, and the display can be more readable when riding because you can show lesser values. Both are fine, but the Joule differs in that you have 6 items on he screen, but when you highlight the values, it shows you associated values. . .so in essence it's like having 18 values at your fingertips. Garmin mount is slick (nice), but the buttons are on the side, so when you're tired and trying to press a button (like the interval button when you're shaky after a shattering interval) good luck not taking the device off the mount by pressing the button, or worse yet turning the device off as you try to grip the device hard enough to press the buttons -- crap design.
Buttons - JOULE: Garmin buttons are hard to press, and the Joule joystick is a pleasure to use. The joule interval and mode buttons are great in that it's really easy to press them when doing a hard interval. Sometimes the Joule joystick button gets depressed when you're trying to move it up or down or left or right, but it doesn't cause any catastrophe, unlike the garmin where you can take the computer off its mount, or turn off the device and hose your ride data when trying to see something.
User interface - JOULE: the garmin has 3 screens, and you can customize each, but the Joule has amazing versatility, and oddly enough, the things that I didn't think I'd care to see I've found really work once I started training with the Joule. e.g. seeing a list of intervals, with avg wattage, time, distance, etc. is great to see when to knock off your intervals because power output has decreased. The concept of highlighting an item in the main display is great, as you can quickly navigate to values that you want to see.
Features - tie, but JOULE (for training): garmin has GPS, so elevation and speed will be more accurate (the wheel circumference is calculated each ride - yes, each ride - and elevation is somewhat correlated with GPS data, and can be fixed post-ride with topo info), but there are some huge advantages to the Joule's data analysis while riding. . .for instance, I thought I'd never want to see my mean maximal power output while riding, but realized that this is exactly what I needed during a tempo or threshold ride to spur me on to keeping up the wattage to match another workout or time period (let's say the last month). When I'm doing sprint training, I can also easily see how many times I broke over a certain w/kg. . things I usually had to figure out after the ride. At first I was dubious about whether I'd use this, but find that I actually do look at these mid-ride to make the best use of my very limited time. And if I was trying to do a 8X1, I never could tell if did the right number of intervals with the Garmin, because I also used the interval button to indicate the end of my warmup and sometimes it got hit twice as the buttons were bad, so I couldn't rely on the count. The joule lists all of the intervals with details like time and avg watts, so it's easy to tell.
So I like the Joule much better.
Best of all: it doesn't crash and lose all of my data!
Some differences in how you ride with one as well:
* The garmin takes a while to get satellite data, and you have to start the timer, so you tend to turn it on, go fill up your water bottles, and ride, whereas the Joule starts running the clock as soon as you start either heart rate or movement (settings driven), so I power on the Joule when I am already riding and want to start the clock.
* Intervals have a GREAT deal of support on the Joule: you can see a list of the intervals you did, with details, during the ride, and you can set the main display to show you stats for only the current interval by holding down the interval button for a few secs. This feature set is done right, and makes this a simple and great training tool when training with power.
* the power smoothing is different - in Gaarmin you choose to show either instant power or a 3-sec rolling average, in the Joule you set a rolling smoothing seconds setting, and it affects everything. At first I didn't think I'd like this, but I find with 3-sec rolling engaged, it's fine.
* It keeps ride history in separate summarized history files, so you can download and delete the ride files (with second-by-second data), but you can keep a year's worth of summaries on the device, so you can easily see how that killer 20-min FT interval you just did stacks up against the last 2 weeks, 4 weeks, 8 weeks, or whatever - and pretty easily while you're riding.
* during group rides or really hard rides when I'm not looking at the meter, it's nice on a descent to see how the first hour of your ride has gone, or how hard you are pushing by looking at TSS and IF scores mid-ride. . .again I didn't think I'd care, but I've found that on a half-century tempo ride this can help to adjust my intensity for a desired effect. Same with NP, which you can again see on the Joule. .. none of this on the Garmin.
Again, most important is that the Joule actually functions: I went through 3 Edge 500 devices and none of them worked with a powermeter with enough stability, and their tech support is below abysmal. I called Cyclops with a problem and they got me in touch with a developer right away -- very different level of interest in their customers.
The Joule with heart rate strap was 450 with a discount, and I bought a speed sensor (it reads cadence off my quarq) for about $30, so all in about $500. The garmin was $350 with speed/cadence and heart rate, but since that stuff never worked anyways, I don't feel like I got a worse deal.
Joule: thumbs up - I'd buy one for a friend.
Garmin Edge 500: only buy it if you ride for fun, don't measure your data or mind if you loose data, and probably not a good idea if you have a powermeter as the interval button freezing bug will be a real killjoy. If you like GPS data, though, it's for you!