I think the Joules are probably really fantastic units, but as a someone who races a lot, I've been very happy with the Garmin 500. On the plus side, I like the elevation numbers, GPS and small size (if I'm trail running, snowshoeing or xc skiing off season, I just stick it in a bag in my pocket, strap on a HR strap and get good data also), plus the GPS data is decent and handy when I'm on trails or crossing unnamed peaks or ridges (no map, but I prefer the small size, and I can reference the GPS map overlay when I download later). Plus, again it's small, so it's a no brainer to leave on for races.
I've had it freeze up a twice in a year... primarily when I was pressing the lap button a lot for <1min intervals - I don't do this anymore (there's probably a way to pre-set a workout like this, but I like to keep it simple). The freeze-up was frustrating, but a 4-button power-down/power-up fixed the issue both times. Your instant power (or 3sec power) and 30sec power are usually sufficient for short interval data. If you're doing longer intervals, current and avg power for the current lap works just fine. There are at least 3 screens you can set up (I think there are more under the "workout" option, but I haven't gotten into that) and you can have a few large data boxes, or 8 small ones.
In a nutshell, it's not perfect, but it's amazingly versatile, and very useful for someone who doesn't need to be inundated with data. Realistically, it doesn't matter a hill of beans whether your TSS for one specific ride was 99 or 101, unless that's your thing. If I'm doing a set of intervals at, say 300W, I keep that in mind, accomplish them around 300W, and then file the data with all the other stuff. Reviewing the minutia mid-ride just isn't my thing, so the 500 is perfect for me.
spiffo - this was the same issue that I had - hitting the lap button. Say I did 5x10m intervals: by the time I hit the lap button 3 or 4 times my garmin would freeze. . but this was supposed to have been fixed in a firmware update, so try updating yours as this shouldn't happen anymore?
I don't look at TSS, but being able to review my intervals during a ride to see how close I am to the percentage drop that would cause me to see that I'm out of the suggested range for the workout per Allen / Coggan protocol, among many other things I posted originally, are the kinds of things that the Joule does that the Garmin 500 does not