I've seen express boarding about 50% of the time since I started making weekly trips between MKE and DFW in early May. These flights are on the 76 seat ERJ-170 and are nearly always full. It's not consistently offered or enforced depending on the station and GA. I've taken advantage of it a couple of times, but made status yesterday so it's technically a moot point for me until I lose status.

Part of the express boarding strategy is to limit the use of overhead bin space for rollaboards by forcing PAX to put smaller items under the seat, thereby reducing last-minute gate checks. When the GAs and FAs manage the process, it works as planned - the last PAX boarding with a rollaboard can find space for it, although the PAX who want to put smaller items in the overhead can get distressed about being told they can't use the bins - and I've seen a few become verbally abusive toward the FAs.

I haven't noticed a significant difference in boarding duration when it's used vs. not used, but I wasn't exactly doing time-in-motion studies with a stopwatch, either. It would be interesting to see if the airline has captured any metrics to determine the effectiveness of this change.