I originally built my REEB with a belt. I bought it as a frame and they included the system for a reasonable price.
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But I eventually went back to a chain.
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It came down to a few reasons.
In the hot desert the system is less quiet with sand in it and the dryness tended to cause it to squeak like a, well, a dry belt. It's easy to silence with a squirt of water, though.
They say it's reliable and I had no belts break but I bikepack with it. I couldn't bring myself to do longer tours without a spare belt even knowing the risk was very small. My luck is if there's a 0.1% chance, though, it'll happen to me.
You can coil the belts on to themselves if you're careful into maybe an 8" disc but you need to be very careful about kinking them. They come in a big box that looks like you're getting a head gasket. Gates has a page that shows you the trick to twisting them into IIRC three concentric circles without breaking it. It's important to do it right since a crease or fold will ruin them and so even twisted finding a place for the spare was a pain. Plus belts aren't cheap. But in fairness I rode the original belt nearly two years without an issue other than the occasional squeaking and never needed the spare, so I do think it's reasonably tough.
The final straw was that sand *did* wear out the front ring very fast. The rear cog is stainless but the front is aluminum, which when the anodizing wears off wears out real quick. When I went to price it just a replacement ring was more than buying a regular ring, chain and cog. I didn't love it enough to spend that much beer money.
Sold both my worn and unused spare belt and still good rear cog for a bit less than a Wolf Tooth front ring and Chris King cog cost. Both of those will last forever with $15 SRAM chains once in a while.
So while the system is trick and serves a niche (would definitely consider them on commuters and wet places) it didn't scratch the itch for me long term.
