It can be but it needn't be. The Canadian/US agreement for preclearance facilities gives Canada the right to establish facilities in the US, although to date it has not taken advantage of this right. The UK/French reciprocal agreement with Eurostar works quite well, too, with the idea being that it's more efficient to deal with passengers as they arrive over a somewhat spread-out amount of time instead of dealing with them *en masse* when they all simultaneously arrive.

As to whether it's good or bad for travelers, it all depends on how well it's done. Personally I prefer clearing Customs at the beginning or the end of my trip, if I can. It prevents me from needing elongated periods of time to make a flight connection, and relieves me of stress. Whether I clear US Customs and Immigration in Toronto/Calgary or Denver/Minneapolis is not a huge issue to me since I'd be connecting anyway.