Well, I've never known any riding ones...in my area they're all driving/heavy plow horses. Clydes, in my experience, are pretty quiet and good natured; they're a lot more easy-going than Percherons (which my dad has =\ ). They're pretty much the ideal of heavy horse-slow, patient, quiet and hard working...the ones around here, anyways. I don't really want to know what the show-bred ones are like! As far as heavy breeds go..yeah, I like them. They're not my favourite breed, but I certainly have nothing bad to say about them as a whole.
I ride a Clydesdale and am planning on purchasing a Clyde cross soon. I've found the ones I know are very good tempered and always want to please you. They know they can easily push you around but it doesn't seem like that's on their agenda.
They are my favourite large breed but I haven't met many drafts.