Can Siri be used effectively by someone who stutters when they speak?
It’s hard to answer definitively, because it greatly varies depending on the degree to which the person stutters.

I have a stutter, though it’s fairly mild these days thanks to a few decades of working to correct it. I do make a conscious effort to form my thoughts completely before speaking to Siri, and I’ll still stutter on occasion. Sometimes the result is workable, because Siri doesn’t need to recognize exact sentences, but instead relies on general meaning and keywords; other times I just have to start over.

It happens infrequently enough that it doesn’t bother me. Now if you stutter a lot it might become frustrating, but that depends on your patience level, and once again on the extent to which you stutter. And even someone who stutters “a lot” may do so in a way that is easy for a machine to correct, if that makes any sense.

As an aside, I think most people have to form complete thoughts in their heads before speaking to an AI. The state of the technology doesn’t enable it to deal with what would be considered “normal” speech patterns anyway, with all its hesitations, self-corrections, etc.

From my limited knowledge of speech impediments, different people struggle with different phonemes, certain sounds in specific arrangements, etc. Myself, I struggle greatly with the “W” sound at the start of a sentence (among other things). This is highly workable with Siri, because if I can get past the very first letter of a question, then I’m usually good to go 

For the most part, interactions with Siri are fairly short. “What time is it?” “Shuffle my Favorites playlist.” “Give me directions to Starbucks.” I’d say if you have reliable mechanisms/tricks for working around your hurdles in short bursts, you’ll find Siri pretty usable.

Sorry for all the “It depends,” but hopefully this helps at least a little. There is solace in that Apple has been focusing on Siri’s error rate, and accessibility is an area of importance to Apple.

And quite frankly, even native speakers with no speech impediment complain about Siri’s error rate all the time  I’m just trying to say there is going to be a baseline amount of frustration with the technology no matter what.

I would recommend relaying your experience with Siri to Apple. Also Tim Cook appears to read all emails he receives, though he may not always respond. He cares deeply about users with accessibility needs, as he has made evident on numerous occasions. I’m not going to post his email address here, but it’s easy to search for.