It's the best out there, sort of, for getting an accurate number of miles, anyway. And depending on the modules you buy (Streets, HAZMAT, Fuel Optimization, Tolls, POI's, Zip Codes, etc), it is the best there is. Somewhere around 85% of all trucking companies use PC Miler, as does the IFTA and DOT, other agencies.
It's also ridiculously overpriced (which is why it's nearly impossible to find out the pricing anywhere online). The individual modules alone cost more than many standalone GPS units. From a purely cost effectiveness analysis perspective, you'd be hard pressed to haul enough freight to justify the cost of an $800 Garmin, but such a unit has many other benefits, tangible and intangible. You'll never haul enough freight to justify the cost of a full-blown PC Miler for an individual truck. For carriers, it's priced according to how it's to be used, number of trucks, lots of factors.
Basic PC Miler, which gives you Zip Code to Zip Code mileage, is $895 for the single license owner/operator version. If you want door-to-door directions using actual street addresses, add the "Streets" module $1000. If you want the "HAZMAT" routing module, another $1000. It gets real ugly after that. For Fuel Tax reporting and basic mileage, though, many truckers can get a lot of use out of PC Miler. For them it's more cost effective. For most, a good Garmin or Tom Tom and dirt cheap eTrucker software does the same thing.
A medium sized carrier, however, depending on the modules and how it's all integrated into their system, could pay half a million dollars or more to have PC Miler installed into their system, and it would be very cost effective for them to do so. For an individual trucker, not so much.