Why does whip cream dispenser sometimes produce "ragged" cream?
When I first got my iSi I had wildly inconsistent results, including exactly what you're describing.  I'm not sure if you're just whipping cream or if you're whipping other cream-based preparations, but the most common reason for this happening is that the liquid going in isn't quite, er, liquid.  Any solid particles of any kind will almost assuredly cause sputtering, which is why they recommend that you strain everything through a fine sieve.

Other things I've learned about the iSi whippers to help prevent these inconsistent results:


Ignore everything the instructions tell you about how to charge it.  They're somewhere between very misleading and complete nonsense.  What I actually do is this:


Screw on a charger as described.
Shake it vigorously, as if it were a cocktail shaker.
Let it rest for a moment, then shake again.  Repeat the shaking 4-5 times.
Finally, remove the charger and screw the cap back on.


The instructions are full of ominous warnings about not being able to discharge it properly if you shake it too much because the nozzle will get blocked or something.  Never happened.  What's far more likely is that the cream doesn't actually whip fully or there are large air pockets.  Do not err on the side of caution here; many "external" iSi recipes such as those you find in the HRC actually tell you to shake frequently as the dispenser chills.  I'm not sure if I'd do this with cream (as opposed to a water-based foam) but I can't stress this enough, don't skimp on the shaking.
If you need to chill it some more after charging, make sure to chill it on its side.  The instructions are quite explicit about this and in this case they're actually correct.
Shake it again after you chill it and before you dispense any.  This is very important and conspicuously absent from the instructions.  You shouldn't need to shake as much as the first pass, but some of the gas does seem to separate over time.
Start by dispensing the cream very, very slowly, until you actually see some come out, then you can apply more pressure as needed.  If you squeeze the trigger as hard as you can right off the bat, you'll just end up depressurizing the thing before any cream has a chance to come out.  Every single time I've pulled too hard, I've ended up with spatters.
Finally, make sure that you actually thoroughly clean it all between uses, including the tiny horizontal hole in the metal tube that you screw the tips onto (I forget what it's called) as well as the socket that piece goes into.  Any blockages whatsoever, no matter how small, can cause at least minor sputtering.


Hope some of that helps.  If you do all of that, you shouldn't have any sputtering issues.  Last 7 or 8 times I've used mine, I haven't had any.