Two years with a 520, approaching two years with an Elemnt Bolt. Hardware-wise, they're both fully competent units. The differences in the software are almost incomparable. I've had the Garmin freeze on the launch screen so many times, I can remember the horrific, convoluted process that needs to be done to fix it-- which involves a series of "hold this button" while plugging it into a PC. The Wahoo has never even been in the house. It does everything via Bluetooth, and 95% of it's programming is done through the phone app. The buttons work better. The battery lasts longer. It never randomly eats rides, or has what I term "GPS strokes" in which the Garmin momentarily loses the satellite lock, and defaults to zero time, sometimes adding 30 hours to a ride time. The Garmin did this so often, I kept fitfiletools.com on my bookmarks bar.

Restart in the middle of a ride without warning? Yeah, a Garmin will do that. Work absolutely perfectly for 3 months, then quietly update its firmware and stop working entirely? Oh yeah. Fill up with .gpx files, never tell you, and start crashing randomly, needing to be put into Mass Storage Mode and cleaned manually, via a computer? Absolutely. Garmin software is pretty much terrible. The Wahoo does everything user-related better, faster, and more intuitively. From the very first ride that I clamped the Wahoo onto the bars, I said "this feels like a bike computer that was made by people who have actually ridden a bicycle." And I still feel the same way now, literally hundreds of rides later.