Delta and Hawaiian are the two US operators. Hawaiian loves it, and is struggling to replace it. They use them for island-hopping. Modern airliners don't do well in this use case — modern engines have been engineered to tight tolerances to extract fuel economy, and their thermal designs rely on the cold air at cruise altitude to keep them healthy. Island hops are too short and too low for that to hold. The engines on the 717 handle it just fine, but anything else on the market will require the planes to sit on the tarmac for a significant length of time between landing and takeoff for them to cool enough, and would destroy Hawaiian's current network design, which has lots of quick flights and tight turns.

Delta announced retirement a long time ago, but has been not-so-secretly buying up as many as they can get, mostly as spares for parts to keep the fleet going because it's so useful for them. Also, the A220, its replacement, has a single engine, the Pratt & Whitney Geared Turbofan, which has had an incredibly rough entry into service — everyone was concerned that the gearbox would be an issue, but it has been mostly rock-solid, and it's the engine core that's had dozens of issues. An airline in India has over half of its fleet grounded and filed for bankruptcy last month because P&W can't repair their engines.