Mules are awesome, and are actually preferable to horses on several counts. Generally, mules can exist on lesser food and won't eat themselves to death like a horse will, their feet are usually very good, they are sure-footed, they think about things instead of just bolting like a horse does and they are sturdier and more heat-resistant. You do have to convince them to do something--partner with them, if you will--instead of just telling them like you would a horse. A mule's self-preservation drive is high and they are very intelligent, which some people say is "stubbornness."
That donkey is amazing! I have seen big mammoths, but he takes the cake! Our donk is a large standard (just under mammoth height), but he has the personality of any six horses. Everyone should have a donkey! Meredith Hodges of Lucky Three Ranch events on her donkey and trains mules for dressage and other disciplines and does a very nice job. She has a series of mule- and donkey-training guides that may be helpful to you.
I beg to differ on the underlined quote. I have only known 4 or 5 mules and never have owned one, but at least 1/2 of them were much spookier than my horses (which are/were Arabs and a Mustang).
I always hear the quote that they are self preserving and not spooky, but I have not found that to be true in my limited experience. The spooky mule I rode was very affectionate though and had wonderful, wonderful feet. I did love the tough, bare feet she had. She was also big and well built. But spooky. Too spooky for me to relax on her. But she was awfully sweet.
I have also known two other spooky mules. So I dunno, I know they have their following, but I kind of think they are a bit over-rated.
As for gaits, the molly mule I rode had a nice fast, smooth walk, much like the "flat walk" of a gaited horse. No need to get used to it, it felt great to me! Her trot swung me side-to-side but was not any rougher than my horses. I rather liked her trot.
The one thing I would have to get used to though, is even though she was built like a tank, she was narrower in the withers than a horse. So you feel like you are riding on a narrow animal even though she was nice and broad in the chest. I think the narrow wither area is a conformation trait of mules (but mule folks can correct me if I'm wrong on that).
So that is my personal experience and opinion on mules. I also think that if the seller is telling you she is "strong willed" she is trying to warn you in a round-about way that the animal is difficult to work with. If she wasn't, why even say so?