Sorry, can't agree!
I actually find Jim Dale's readings of the Harry Potter books to be superb. He doesn't so much read them as perform them. He establishes a particular voice and inflection for every character and is amazingly consistent in going back to that voice every time the character speaks (if you've ever had an enthusiastic parent try and read you a bedtime story in this manner, or tried to do it yourself, you'll know how hard this is to pull off). I especially like his voices for Hagrid and Hermione. I'm not sure what being American has to do with enjoying it, as Jim Dale himself is British. Why they use different narrators on either side of the pond is a mystery to me. I've never heard Stephen Fry's readings, but I'm sure he's excellent as well.
Jim Dale's penchant for voices reminds me of Peter Dennis' excellent performance of A.A. Milne's ("Winnie the Pooh," "The House at Pooh Corner," etc.) books. He also assigns wonderful voices to all the characters and sticks with them throughout. When readers do this, you start to forget it's one person reading and can easily begin imagining these characters actually talking to each other and to you (especially invaluable when it is something that will be enjoyed by children). It's an amazing talent.
I'd also like to recommend Martin Shaw's unabridged reading of "The Silmarillion." The mere ability to successfully pronounce the littany of people, places, and things in Tolkien's mythology deserves a medal. But Shaw also successfully speaks Elvish and generally delivers a compelling reading of dense material as well.
I have great respect for opinions, however much I may disagree with them. I don't mean to cast doubt on the above quoted post, so keep in mind that this is just my own humble opinion, with a sprinkling of past opinione that have been expressed to me.
In short, if Stephen Fry had not provided his own version of J K Rowling's wonderful series, I would still be casting this first viewpoint. Keeping it simple, Jim Dale has an extremely annoying impediment. He reads the books as though he is in some kind of hurry to get to the end. This is not just about the speed at which he reads, in fact it's more about the mono toned manner that uses, which shows a total lack of interest in the passages he reads.
Okay, now for a more direct comparison to Stephen Fry. In Britain Stephen Fry is recognised as one of our absolute best orators. He is an actor, a director, a TV presenter, a documentary narrator, a gifted MC, and an incredibly popular chat show guest and all round personality. I'm sure he is also in great demand as an after dinner speaker. His diction and general knowledge is second to none, and is globally very underated. All of this will probably have some bearing on his ability to read a story, and read it well. I have heard it said that people in the US enjoy Dale's characterisations, and has even received awards for his Harry Potter work. I would say, however, that this is going to be largely because of the high profile nature of the books, and the lack of competition n the US reading this series. I'm not saying he's bad, I'm just saying that he's absolutely, without any doubt, not a patch on Stephen Fry. Some of Dale's characterisation's are good, but ALL of Stephen Fry's are first class. The Weasley twins are an ideal example. Jim Dale gives them a London accent, that takes away some of the extremely witty dryness that Stephen Fry gives them. It's as if Jim Dale was oblivious to the fact that the Weasley family are from the West Country! They are just so much funnier when Stephen Fry brings them to life. In fact, all the characters come to life in much the same way. One final note, Stephen Fry is J K Rowling's personal favourite reader. By the way, I wonder if anyone noticed Stephen Fry's role in the US TV series 'Bones? For a really good example of his acting skills, see him take on the title role in the movie, 'Wilde'...which is of course a movie biography of Oscar Wilde!
I absolutely urge anyone who is wanting to listen to an audiobook version of this series, to go with Mr Fry's reading. If you've already listened to Jim Dale's reading, and plan to reread the books, try out Stephen Fry. I can't promise that all of you will be converted, as everyone has their own opinion, but I can say that about 95% of people who I know who have listened to both readers, prefer Stephen Fry!