Same rules as apply to .ca in english, has to be a good, commercial keyword or have traffic - there is a french market in Canada to market to!
Some of my best traffic is to my french .ca's with even simple development.
Pros are:
less competition from other french sites. with english .ca you have a bigger market, but also heavy competition from the US and other english sites if you decide to develop.
you will also get France traffic and other traffic from small countries with french language.
you can be a bigger fish in a smaller market.
you can use Ad sense to monetize (it will show french ads on pages with french text), and there are more french affiliate programs coming out all the time (like on Clickbank and CJ).
easier/cheaper to acquire good keyword domains. for example I've hand regged or paid <$70 in TBR for the french equivalent of english words like "poem, poems, free, big, nuts, onlinegames, pear, celebrity, girlgames........"
many domainers are english and don't bother with the french market, or can't get into it due to language barrier
Cons are:
french is a smaller market in Canada and the world compared to english, but like mentioned above there is less competition for it.
less monetization opportunities. there are many affiliate programs etc. I'd like to use but they aren't in french, but like mentioned above it is getting better
less resale value, don't think I've ever had a really good offer for a french .ca compared to my english ones.
So yes, all in all good french .ca can be worth developping, I think a quarter to a third of Canada speaks french and you can't reach most of these people with english domains.