I have used Express Pool quite a bit in San Francisco, here are my comments.

Pickups are challenging. The Uber app might say, "walk to NW corner of XXX and YYY street." But the driver may end up approaching the SE corner. Or the entire intersection will be clogged with traffic and have no place for a car to pull over.

Dropoffs are confusing. You won't know where the driver is actually taking you, because the dropoff location may change as co-riders are added. You just have to wait for the driver to tell you when it's time to get out of the car, and then you're on your own to your actual destination.

Savings are trivial for short trips. A 2-3 mile ride from SOMA to Hayes Valley just now was $5 Express, $6 Pool.

Savings may be substantial for long trips, but is your destination walkable? I often see SF to South Bay on the order of $20 Express, $30 Pool, $40 UberX. But beware that suburbs can be impractical to navigate on foot. I had a co-rider who would have had to cross a highway and walk uphill to a gated community after his Express Pool dropoff. Our driver took pity on him and drove to his front door.

Even a short walk could be hazardous. In many areas, 1 city block is all it takes to separate safety from danger. I once called Uber Express Pool from a luxury apartment late at night and saw, "walk to 6th & Mission." Cancel, cancel, cancel!