How often and where do you use them?
On almost every lowD that would be rolled if it were higher 
and sometimes on low E’s and on high D’s…
It’s a matter of taste and depends on what style you like. I’ve heard some whistlers and flute players use them sparingly, I’ve heard a lot of people frown upon any use of the cran outside of piping, and many whistlers don’t use them at all. When I do play them I use them for D and E. I never roll my low E’s, I prefer to use a 2-cut cran instead as it comes out a little sharper and more audible on most whistles. As far as low D and E, unless you’re playing solo, a crann may just be a waste of energy as it’s not likely to even be heard when mixed with other instruments… not that I let that stop me from playing them 
I’d still use them sparingly though, you don’t even hear pipers utilizing them constantly and that’s where they are probably used most. Ornamentation can lose it’s effectiveness if it’s overused.
In a nutshell, a crann can be used in place of a roll. Straycat’s advice is good although I would say it’s no so much a question of “sparing use” but rather the mood and effect that one is trying to illicit in a given tune. Needless to say, proper execution is a prerequisite.
Some players, like Mary Bergin, almost never use them. I almost never used cranns, until I started to play the pipes. Now, there are certain tunes that I always play with cranns on pipes and whistle like Garrett Barry’s, Humours of Ballyloughlin, and Dusty Miller (I find that they fit in with the jig rhythm easier for me than reels). It’s true that in session play, they get lost. But those times when it’s just you on whistle with a piper or a fiddle player, it adds a wonderful rustic touch. I also use it on low E on tunes like My Love in the Morning as well the high D in some tunes. But too much and you sound like a piper who’s playing the whistle. On the whistle, you do need to vary it with a “dedd”, and a high E roll often sounds nicer on a whistle than a high E crann.
Use it sparingly and use it in proper time.
Ditto the previous posts, particularly Gabriel. Also:
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It’s possible, if not common, to cran any note except B and C/C#. A good example is Joanie Madden’s air “The Immigrant”, where she uses high f# crans to good effect at that slow tempo. I find top hand crans on G and A hard to execute cleanly – the Dr. Strangelove effect.
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Instead of cranning D/d, I sometimes substitute a “Micho Russell roll”: e.g. |D{F}DD| instead of |~D3|, with the last D tongued to simulate a tap. I call it this because Micho Russell almost never plays rolls in his recordings I’ve heard, but uses this pattern instead.
is there anywhere on the net where i can find a recording of Madden or Russell?
Sorry, I nearly forgot. I don’t know about Joanie, apart from CD samples on Amazon, etc.
For Micho Russell, there are a few nice tune samples here:
http://www.pennywhistle.com/soundtransc.html
And some complete tunes here, with commentary:
http://www.rogermillington.com/tunetoc/index.html
Also see this new thread here:
http://chiffboard.mati.ca/viewtopic.php?t=48115