I'm Police Officer on the Job in Southern Ohio, at my Department we all Carry Glock 22, used to carry whatever we wanted, but everyone now love the Glock and can't imagine carrying anything else. Also across the Ohio River Kentucky State Police all changed to Glock 22, I thing the 22 rules in Law Enforcement at this time.
I honestly do not know a single officer who carries a Glock for duty use, though a few carry them off-duty. Most agencies in my neck of the woods followed the Michigan State Police and went with Sig P226's. Some have .40's, while others have 9's.
The local sheriff's department had to go their own way, so years ago they wen't with 10mm Smiths, because the 10mm was 1 better than the 9mm. They changed again about 2 years ago to .40 cal S&W Sigmas.
"but they got them for FREE so I don't know if that is a barometer for popularity vs. others. "
Better check your sources. Customs paid for every gun in their inventory under federal contract mandates. They even stopped using seized guns thanx to an Ex. Order from Klinton, except for a few leftover from the old days designated for special purposes.
Inspectors carry 17's as their sidearm, and Special Agents and such use 19's as the primary sidearm, with a 26 as a backup - with a handful of J-frames still in use as backups for the old timers. Customs replaced the 6906 & 6946 series guns about 4 years ago, with which they had terrible reliability problems. And last time I heard, Customs was the 2nd largest armed LE agency in this country - but that info could be a little dated, since they have grown in the last several years.
The policy in my department is that all of the Glock .40cal Models are approved for duty and plain clothes. The department is split between the Glock 22 and the Glock 23 and I myself carry the Glock 35 with the Model 27 as a detail or court weapon of choice.